staging: rtl8723au: 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.

This bug seems to be present in multiple drivers provided by Realtek.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>