faudio: Fix XNA_GetSongEnded false positives

Now that we release buffer lock before executing callbacks, some code
from another thread can get scheduled between mutex unlock and callback
call and observe empty buffer queue right before callback puts a new
buffer there. XNA_Song determines the end of the song by simply checking
if the queue is empty, which may lead to random playback breaks.

Not sure how to properly check for the song end but the buffer decoding
function also zeroes out total samples counter when it finishes a
EOS-flagged buffer so checking if that counter is zero should cut it...
probably?

src/FAudio_platform_win32.c

@@ -1069,7 +1069,7 @@ FAUDIOAPI uint32_t XNA_GetSongEnded()
 		return 1;
 	}
 	FAudioSourceVoice_GetState(songVoice, &state, 0);
-	return state.BuffersQueued == 0;
+	return state.BuffersQueued == 0 && state.SamplesPlayed == 0;
 }
 
 FAUDIOAPI void XNA_EnableVisualization(uint32_t enable)

src/XNA_Song.c

@@ -325,7 +325,7 @@ FAUDIOAPI uint32_t XNA_GetSongEnded()
 		return 1;
 	}
 	FAudioSourceVoice_GetState(songVoice, &state, 0);
-	return state.BuffersQueued == 0;
+	return state.BuffersQueued == 0 && state.SamplesPlayed == 0;
 }
 
 FAUDIOAPI void XNA_EnableVisualization(uint32_t enable)