wlan: Kernel crash from freeing bss list

Data abort while traversing bss link list as next pointer is
NULL.Therfore we should reset ''pMac->ft.ftPEContext.pFTPreAuthReq' to
NULL, causing the FT session object not to
be checked for freeing in 'limFTCleanup'

Change-Id: I70eba6f1619f2d51bddf248f9ecf7a741586bf30
CR-Fixed: 436485
(cherry picked from commit 2d335f3ff97a4da06e1c3d6d4365ca5c2e2d49ec)