staging: r8192U: Fix check pointer after usage problem

In routine rtl8192_tx_isr(), pointer skb is dereferenced before it is
checked for NULL. This patch has only been compile-tested, as I do not
have the hardware. This problem was reported at
https://bugzilla.kernel.org/show_bug.cgi?id=109951

.

Fixes: bugzilla.kernel.org: #109951
Reported-by: Yong Shi <brave_shi@163.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Yong Shi <brave_shi@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>