fix several errors/warnings which only appear with -O3

git-svn-id: https://svn.musicpd.org/mpd/trunk@7333 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/decode.c

@@ -342,6 +342,8 @@ static void * decoder_task(void *arg)
 			decoder_sleep(dc);
 		}
 	}
+
+	return NULL;
 }
 
 void decoderInit(DecoderControl * dc)