Staging: rtl8192u: r819xU_cmdpkt: checking NULL value after doing dev_alloc_skb

Checking the return of dev_alloc_skb as stated in the following bug: https://bugzilla.kernel.org/show_bug.cgi?id=60411



Signed-off-by: Iker Pedrosa <ikerpedrosam@gmail.com>
Reported-by: RUC_Soft_Sec <rucsoftsec@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>