qcacld-3.0: Fix Static Analyze defect in sme_qos_ft_handoff_required()

In function sme_qos_ft_handoff_required(), CSR_GET_SESSION() is
possible to return NULL pointer, so add check before de-reference it.

Change-Id: Iee4e1d03da550969b4da1b746e4325f59ab72146
CRs-Fixed: 2669223