decoder_api: always notify_wait() for free chunks

One of the previous patches made MPD consume 100% CPU in a busy wait:
when the music_pipe was full, it did not wait (with notify_wait()) for
free chunks, because a variable has a different meaning now.  Always
pass "true" as the "wait" parameter.

src/decoder_api.c

@@ -243,8 +243,7 @@ decoder_data(struct decoder *decoder,
 		if (dest == NULL) {
 			/* the music pipe is full: wait for more
 			   room */
-			enum decoder_command cmd =
-				need_chunks(is, nbytes == 0);
+			enum decoder_command cmd = need_chunks(is, true);
 			if (cmd != DECODE_COMMAND_NONE)
 				return cmd;
 			continue;