LinuxKPI: 802.11: initalize lsta to NULL in linuxkpi_ieee80211_rx()

The logic following this new assignment in some cases may not set lsta
but the later tracing is checking for it to be != NULL.
With lsta not initialized that check may not hold up and later we would
dereference lsta->state and possibly panic.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D45507

(cherry picked from commit 582469016aed4fac3a7ead24dc31000edbb7e823)