As the nla_nest_start() may fail with NULL returned, the return value needs
to be checked.

Fixes: ce08cd34 ("wifi: nl80211: expose link information for interfaces")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Link: https://lore.kernel.org/r/20221129014211.56558-1-yuancan@huawei.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>