Check for the fi parameter to be non-NULL before trying to fetch the
ssi_signal information similarly to how the fi->freq was already
handled. While the meta information is supposed to be available, it
looks like there is at least one corner case where fi == NULL could be
used (Authentication frame reprocessing after RADIUS-based ACL).
Signed-off-by: Jouni Malinen <j@w1.fi>
659ac96d7f