PlatformPlayer: Only stop media playback if it is running

"Orcs and Elves" constantly tries to stop a MIDI stream that isn't
even running, and ends up causing a stack overflow on
PlatformPlayer. Since it only makes sense to stop streams that are
actually running, add a check for cases where they aren't.

src/org/recompile/mobile/PlatformPlayer.java

@@ -238,6 +238,7 @@ public void start()
 
 		public void stop()
 		{
+			if(!isRunning()) { return; }
 			midi.stop();
 			state = Player.PREFETCHED;
 			tick = midi.getTickPosition();
@@ -341,6 +342,7 @@ public void start()
 
 		public void stop()
 		{
+			if(!isRunning()) { return; }
 			wavClip.stop();
 			time = wavClip.getMicrosecondPosition();
 			state = Player.PREFETCHED;