wlan: Fix possible null pointer dereference in WDA_IsSelfSTA

In function WDA_IsSelfSTA(), pointer pWDA will be NULL
dereferenced if VOS_GET_WDA_CTXT() returns NULL.
Added NULL pointer check before accessing the pointer.

Change-Id: I40dcac790d0bc76939c13b4af864139889403a02
CRs-Fixed: 679406.