Use main thread of current Lua state for callbacks, when known

If the library is opened from a Lua thread (coroutine) that is not the main
one, the stored L would be to that thread, which may become suspended (by
calling yield). It is unsafe to call functions on suspended Lua threads, which
luv does a lot (the stored L is used for callbacks).

The main thread of a Lua state can never yield, so it is always safe to call
callbacks on.

This commit ensures the main thread is stored in the luv context instead of
the current thread, when we are able to calculate it (LUA_RIDX_MAINTHREAD was
added in Lua 5.2, and I have not found a way to determine the main thread in
Lua 5.1).

This fixes #503

src/luv.c

@@ -835,6 +835,15 @@ static int loop_gc(lua_State *L) {
 }
 
 LUALIB_API int luaopen_luv (lua_State* L) {
+#ifdef LUA_RIDX_MAINTHREAD
+  // Lua 5.2+ - resolve the main thread of the current Lua state, even if
+  // we were loaded from a different thread (which may become suspended/dead).
+  lua_geti(L, LUA_REGISTRYINDEX, LUA_RIDX_MAINTHREAD);
+  lua_State* ctxL = lua_tothread(L, -1);
+  lua_pop(L, 1);
+#else
+  lua_State* ctxL = L;
+#endif
   luv_ctx_t* ctx = luv_context(L);
 
   luaL_newlib(L, luv_functions);
@@ -862,7 +871,7 @@ LUALIB_API int luaopen_luv (lua_State* L) {
     lua_rawset(L, -3);
 
     ctx->loop = loop;
-    ctx->L = L;
+    ctx->L = ctxL;
     ctx->mode = -1;
 
     ret = uv_loop_init(loop);