music_pipe: continuously check the sample format of all chunks

Provide a debug function which asserts on the sample format of all
chunks.  Call this function in each iteration of the player main loop.

src/player_thread.c

@@ -344,6 +344,12 @@ static void do_play(void)
 			}
 		}
 
+#ifndef NDEBUG
+		music_pipe_check_format(&play_audio_format,
+					player.next_song_chunk,
+					&dc.out_audio_format);
+#endif
+
 		if (decoder_is_idle() && !player.queued &&
 		    pc.next_song != NULL &&
 		    pc.command == PLAYER_COMMAND_NONE) {