gtk: Fix memory leak.

free buffer in error path.
Found by clang scan.

Change-Id: I7c271b74e3e5ac9d4f2a550f3c83988a4594133d
Reviewed-on: https://code.wireshark.org/review/25416
Petri-Dish: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>

ui/gtk/uat_gui.c

@@ -264,6 +264,7 @@ static guint8 *unhexbytes(const char *si, guint len, guint *len_p, char** err) {
 
 on_error:
 	*err = g_strdup("Error parsing hex string");
+	g_free(buf);
 	return NULL;
 }