staging: rtl8188eu: core: fix null dereference on exit path in rtw_mlme.c

If adapter is null, null dereference may occur. This patch fixes it.
The following Coccinelle semantic patch was used to find the case.

@@
expression E, E1;
identifier f;
statement S1,S2,S3;
@@

* if (E == NULL)
{
 ... when != if (E == NULL) S1 else S2
     when != E = E1
* E->f
 ... when any
 return ...;
}
else S3

Signed-off-by: Jiayi Ye <yejiayily@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>