prima: Possible dereference of NULL pointer

csrNeighborRoamCandidateFoundIndHdlr calls CSR_GET_SESSION
to get pSession and further calls csrScanFlushSelectiveSsid
to start a fresh scan cycle using parameters associated to
pSession. This could results NULL pointer dereference issue
if CSR_GET_SESSION return NULL.

Fix is to validate pSession before accessing it in
csrNeighborRoamCandidateFoundIndHdlr.

Change-Id: I98221d4e61be7e52fa0cc1b77689708c98a7eeb2
CRs-Fixed: 2303686