staging: rtlwifi: rtl8822b: fix a missing check of alloc_skb

__netdev_alloc_skb() return NULl when it fails. skb_put() further uses
it even when the allocation fails, leading to NULL pointer dereference.
The fix inserts a check for the return value of __netdev_alloc_skb().

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>