mac80211_hwsim: Fix possible null-pointer dereferences in hwsim_dump_radio_nl()

In hwsim_dump_radio_nl(), when genlmsg_put() on line 3617 fails, hdr is
assigned to NULL. Then hdr is used on lines 3622 and 3623:
    genl_dump_check_consistent(cb, hdr);
    genlmsg_end(skb, hdr);

Thus, possible null-pointer dereferences may occur.

To fix these bugs, hdr is used here when it is not NULL.

This bug is found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Link: https://lore.kernel.org/r/20190729082332.28895-1-baijiaju1990@gmail.com
[put braces on all branches]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -3617,10 +3617,12 @@ static int hwsim_dump_radio_nl(struct sk_buff *skb,
 		hdr = genlmsg_put(skb, NETLINK_CB(cb->skb).portid,
 				  cb->nlh->nlmsg_seq, &hwsim_genl_family,
 				  NLM_F_MULTI, HWSIM_CMD_GET_RADIO);
-		if (!hdr)
-			res = -EMSGSIZE;
+		if (hdr) {
 			genl_dump_check_consistent(cb, hdr);
 			genlmsg_end(skb, hdr);
+		} else {
+			res = -EMSGSIZE;
+		}
 	}
 
 done: