commandError() cleanups, fixup gcc checks

stripped binary size reduced by 9k on my machine from making
commandError a function.  We'll print out error messages slightly
slower before, but the smaller binary is more than worth it.

git-svn-id: https://svn.musicpd.org/mpd/trunk@4488 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/player.c

@@ -396,7 +396,7 @@ int playerSeek(int fd, Song * song, float time)
 
 	if (pc->state == PLAYER_STATE_STOP) {
 		commandError(fd, ACK_ERROR_PLAYER_SYNC,
-			     "player not currently playing", NULL);
+			     "player not currently playing");
 		return -1;
 	}