fix crash when post is single line with hashtags (#4778)

```
java.lang.StringIndexOutOfBoundsException
     at android.text.SpannableStringBuilder.<init>(SpannableStringBuilder.java:63)
     at android.text.SpannableStringBuilder.subSequence(SpannableStringBuilder.java:1198)
     at com.keylesspalace.tusky.util.LinkHelper.setClickableText(LinkHelper.kt:99)
     at com.keylesspalace.tusky.adapter.StatusBaseViewHolder.setTextVisible(StatusBaseViewHolder.java:289)
     at com.keylesspalace.tusky.adapter.StatusBaseViewHolder.setSpoilerAndContent(StatusBaseViewHolder.java:244)
     at com.keylesspalace.tusky.adapter.StatusBaseViewHolder.setupWithStatus(StatusBaseViewHolder.java:820)
     at com.keylesspalace.tusky.adapter.StatusViewHolder.setupWithStatus(StatusViewHolder.java:91)
     at com.keylesspalace.tusky.components.timeline.TimelinePagingAdapter.bindViewHolder(TimelinePagingAdapter.kt:100)
     at com.keylesspalace.tusky.components.timeline.TimelinePagingAdapter.onBindViewHolder(TimelinePagingAdapter.kt:82)
     at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7847)
     at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6646)
     at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6917)
     at androidx.recyclerview.widget.GapWorker.prefetchPositionWithDeadline(GapWorker.java:288)
     at androidx.recyclerview.widget.GapWorker.flushTaskWithDeadline(GapWorker.java:345)
     at androidx.recyclerview.widget.GapWorker.flushTasksWithDeadline(GapWorker.java:361)
     at androidx.recyclerview.widget.GapWorker.prefetch(GapWorker.java:368)
     at androidx.recyclerview.widget.GapWorker.run(GapWorker.java:399)
     at android.os.Handler.handleCallback(Handler.java:959)
     at android.os.Handler.dispatchMessage(Handler.java:100)
     at android.os.Looper.loopOnce(Looper.java:232)
     at android.os.Looper.loop(Looper.java:317)
     at android.app.ActivityThread.main(ActivityThread.java:8705)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:886)
```

app/src/main/java/com/keylesspalace/tusky/util/LinkHelper.kt

@@ -137,7 +137,7 @@ internal fun getTrailingHashtags(content: Spanned): Pair<Int, List<HashTag>> {
     return when (trailingContentLength) {
         0 -> Pair(content.length, emptyList())
         else -> {
-            val trailingContentOffset = content.length - trailingContentLength
+            val trailingContentOffset = (content.length - trailingContentLength).coerceAtLeast(0)
             Pair(
                 trailingContentOffset,
                 content.getSpans(trailingContentOffset, content.length, URLSpan::class.java)

app/src/test/java/com/keylesspalace/tusky/util/LinkHelperTest.kt

@@ -317,17 +317,30 @@ class LinkHelperTest {
 
     @Test
     fun `get trailing hashtags with empty content returns empty list`() {
-        assert(getTrailingHashtags(SpannableStringBuilder("")).second.isEmpty())
+        val (endOfContent, trailingHashtags) = getTrailingHashtags(SpannableStringBuilder(""))
+        assertEquals(0, endOfContent)
+        assert(trailingHashtags.isEmpty())
     }
 
     @Test
     fun `get trailing hashtags with no hashtags returns empty list`() {
-        assert(getTrailingHashtags(SpannableStringBuilder("some untagged content")).second.isEmpty())
+        val (endOfContent, trailingHashtags) = getTrailingHashtags(SpannableStringBuilder("some untagged content"))
+        assertEquals(21, endOfContent)
+        assert(trailingHashtags.isEmpty())
     }
 
     @Test
     fun `get trailing hashtags with all inline hashtags returns empty list`() {
-        assert(getTrailingHashtags(SpannableStringBuilder("some #inline #tagged #content")).second.isEmpty())
+        val (endOfContent, trailingHashtags) = getTrailingHashtags(SpannableStringBuilder("some #inline #tagged #content"))
+        assertEquals(29, endOfContent)
+        assert(trailingHashtags.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with only hashtags returns empty list`() {
+        val (endOfContent, trailingHashtags) = getTrailingHashtags(SpannableStringBuilder("#some #inline #tagged #content"))
+        assertEquals(0, endOfContent)
+        assert(trailingHashtags.isEmpty())
     }
 
     @Test
@@ -336,7 +349,8 @@ class LinkHelperTest {
             tags.first().let { append("#${it.name}", URLSpan(it.url), 0) }
         }
 
-        val (_, trailingHashtags) = getTrailingHashtags(content)
+        val (endOfContent, trailingHashtags) = getTrailingHashtags(content)
+        assertEquals(30, endOfContent)
         assertEquals(tags.first().name, trailingHashtags.single().name)
         assertEquals(tags.first().url, trailingHashtags.single().url)
     }
@@ -352,7 +366,8 @@ class LinkHelperTest {
                 }
             }
 
-            val (_, trailingHashtags) = getTrailingHashtags(content)
+            val (endOfContent, trailingHashtags) = getTrailingHashtags(content)
+            assertEquals(30, endOfContent)
             assertEquals(tags.size, trailingHashtags.size)
             tags.forEachIndexed { index, tag ->
                 assertEquals(tag.name, trailingHashtags[index].name)