MidiPlayer: Fix crash when dropping a midi file

Window must be locked or MidiRunThread crashes with a segment violation.

Change-Id: I9d5d0fa477475a9b5ba877aea3d6583690aacb2b
Reviewed-on: https://review.haiku-os.org/c/1080
Reviewed-by: Barrett17 <[email protected]>

src/apps/midiplayer/MidiPlayerWindow.cpp

@@ -427,7 +427,7 @@ MidiPlayerWindow::_StopHook(int32 arg)
 void
 MidiPlayerWindow::StopHook()
 {
-	Lock();
+	if (Lock()) {
 		// we may be called from the synth's thread
 
 		fIsPlaying = false;
@@ -439,6 +439,7 @@ MidiPlayerWindow::StopHook()
 
 		Unlock();
 	}
+}
 
 
 void