playlist: return -1 after assert(0)

print_playlist_result() had an assert(0) at the end, in case there was
an invalid result value.  With NDEBUG, this resulted in a function not
returning a value - add a dummy "return -1" at the end to keep gcc
quiet.

src/command.c

@@ -277,6 +277,7 @@ static int print_playlist_result(struct client *client,
 	}
 
 	assert(0);
+	return -1;
 }
 
 static void addCommand(const char *name,