cfg80211: check reg_rule for NULL in handle_channel_custom()

We may end up with a NULL reg_rule after the loop in
handle_channel_custom() if the bandwidth didn't fit,
check if this is the case and bail out if so.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200221104449.3b558a50201c.I4ad3725c4dacaefd2d18d3cc65ba6d18acd5dbfe@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/reg.c

@@ -2276,7 +2276,7 @@ static void handle_channel_custom(struct wiphy *wiphy,
 			break;
 	}
 
-	if (IS_ERR(reg_rule)) {
+	if (IS_ERR_OR_NULL(reg_rule)) {
 		pr_debug("Disabling freq %d MHz as custom regd has no rule that fits it\n",
 			 chan->center_freq);
 		if (wiphy->regulatory_flags & REGULATORY_WIPHY_SELF_MANAGED) {