wcnss: add NULL check before accessing pnv pointer

Direct accessing of pnv pointer leads for NULL dereference.
To mitigate issue add NULL check before useing a pnv pointer.

Change-Id: I99f96db6a715ad17414572a47ecda1fd55ecceb8
CRs-Fixed: 790447
Signed-off-by: Anand N Sunkad <asunka@codeaurora.org>