wpa: Correctly call pcap_next_ex()

The second argument to pcap_next_ex() is a pointer to a pointer.
Not a pointer. This fixes a wpa_supplicent SIGSEGV.

PR: 263266
Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Fixes: 6e5d01124fd4dd57899ddd9260c76dbb43543aa7
MFC: immediately