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

Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: NAmitkumar Karwar <akarwar@marvell.com>
Signed-off-by: NBing Zhao <bzhao@marvell.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>