staging: rtl8188eu: fix possible null dereference

Inside a nested 'else' block at the beginning of this function is a
call that assigns 'psta' to the return value of 'rtw_get_stainfo()'.
If 'rtw_get_stainfo()' returns NULL and the flow of control reaches
the 'else if' where 'psta' is dereferenced, then we will dereference
a NULL pointer.

Fix this by checking if 'psta' is not NULL before reading its
'psta->qos_option' data member.

Addresses-Coverity: ("Dereference null return value")

Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Link: https://lore.kernel.org/r/20190926150317.5894-1-connor.kuehl@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>