wlan: NULL check to ensure preauth candidate is still valid

STA processes the PREAUTH response from the AP2 after switching
back to home channel(AP1). In this possible timing scenario if the
STA receives DEAUTH from the currently associated AP (AP1) before
processing the PREAUTH response then FTPreAuthReq is cleaned-up
and set to NULL.

This causes the crash in the next immediate processing of PREAUTH.
Fix is to add a NULL check to ensure pre-authenticated candidate
is still valid.

Change-Id: Iba1813e2cd5401c75369c5688b730eb0b5416e90
CRs-Fixed: 565333