wifi: mac80211_hwsim: Fix possible NULL dereference

In a call to mac80211_hwsim_select_tx_link() the sta pointer might
be NULL, thus need to check that it is not NULL before accessing it.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230604120651.f4d889fc98c4.Iae85f527ed245a37637a874bb8b8c83d79812512@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>