hostapd: add missing NULL pointer check on radar notification

Fixes a race condition that can lead to a hostapd crash

Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit d864f68232)

package/network/services/hostapd/src/src/ap/ubus.c

@@ -1907,6 +1907,9 @@ void hostapd_ubus_notify_radar_detected(struct hostapd_iface *iface, int frequen
 	struct hostapd_data *hapd;
 	int i;
 
+	if (!ctx)
+		return;
+
 	blob_buf_init(&b, 0);
 	blobmsg_add_u16(&b, "frequency", frequency);
 	blobmsg_add_u16(&b, "width", chan_width);