wifi: ieee80211: check for NULL in ieee80211_mle_size_ok()

For simplicity, we may want to pass a NULL element, and
while we should then pass also a zero length, just be a
bit more careful here.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240318184907.4d983653cb8d.Ic3ea99b60c61ac2f7d38cb9fd202a03c97a05601@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>