mwifiex: add NULL check for PCIe Rx skb

commit bb8e6a1ee881d131e404f0f1f5e8dc9281002771 upstream.

We may get a NULL pointer here if skb allocation for Rx packet
was failed earlier.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>