Move resumesong assignment to songend()

This fixes a bug where the resumemusic() script command would always
play MMMMMM track 15 (or, if you're using PPPPPP, just not work). This
is because musicclass::haltdasmusik() assigns resumesong AFTER calling
Mix_HaltMusic(), but the songend() callback fires before the resumesong
assignment, meaning resumesong gets set to -1 instead of whatever
currentsong was previously.

To fix this, just move the assignment into the callback itself (I don't
know why this wasn't done before). I could have moved it to before the
Mix_HaltMusic() call, but moving it into the callback itself fixes it
for all cases of the music stopping (such as when the music fades out).

desktop_version/src/Music.cpp

@@ -145,6 +145,7 @@ static void songend(void)
 {
 	extern musicclass music;
 	music.songEnd = SDL_GetPerformanceCounter();
+	music.resumesong = music.currentsong;
 	music.currentsong = -1;
 }
 
@@ -265,7 +266,6 @@ void musicclass::fadein(void)
 void musicclass::haltdasmusik(void)
 {
 	Mix_HaltMusic();
-	resumesong = currentsong;
 }
 
 void musicclass::silencedasmusik(void)