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

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>