fix(tform): restrict sort file creation check to main thread

Restrict the sort file creation check at startup to the main thread in
TFORM. This change prevents unnecessary checks and potential race
conditions caused by multiple threads.

Close #602.

sources/startup.c

@@ -802,6 +802,9 @@ classic:;
 /*
 	Try to create the sort file already, so we can Terminate earlier if this fails.
 */
+#ifdef WITHPTHREADS
+	if ( par <= 1 ) {
+#endif
 	if ( ( AM.S0->file.handle = CreateFile((char *)AM.S0->file.name) ) < 0 ) {
 		MesPrint("Could not create sort file: %s", AM.S0->file.name);
 		Terminate(-1);
@@ -810,6 +813,9 @@ classic:;
 	CloseFile(AM.S0->file.handle);
 	AM.S0->file.handle = -1;
 	remove(AM.S0->file.name);
+#ifdef WITHPTHREADS
+	}
+#endif
 /*
 	With the stage4 and scratch file names we have to be a bit more careful.
 	They are to be allocated after the threads are initialized when there