hostapd: ubus: fix uninitialized pointer

This fixes passing a bogus non-null pointer to the ubus handler in case
the transition request is rejected.

Signed-off-by: David Bauer <mail@david-bauer.net>

package/network/services/hostapd/patches/600-ubus_support.patch

@@ -520,7 +520,7 @@
  {
  	u8 dialog_token, status_code, bss_termination_delay;
 -	const u8 *pos, *end;
-+	const u8 *pos, *end, *target_bssid;
++	const u8 *pos, *end, *target_bssid = NULL;
  	int enabled = hapd->conf->bss_transition;
  	struct sta_info *sta;