staging: r8188eu: Calling rtw_get_stainfo() with a NULL sta_addr will return NULL

This makes the follow-on check for psta != NULL pointless and makes
the whole exercise rather pointless. This is another case of why
blindly zero-initializing variables when they are declared is bad.

Reported-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>