midi_sdlmixer: Check playing status in midi_get_playing_info()

This fixes an infinite loop in Buroburo waiting for MIDI playback to
finish.

src/midi.sdlmixer.c

@@ -123,7 +123,7 @@ static int midi_unpause(void) {
 }
 
 static int midi_get_playing_info(midiplaystate *st) {
-	if (!mix_music) {
+	if (!mix_music || !Mix_PlayingMusic()) {
 		st->in_play = FALSE;
 		st->loc_ms  = 0;
 		return OK;