staging: rtl8723au: fix static checker warning

Fix the following static checker warning:

 drivers/staging/rtl8723au/core/rtw_sta_mgt.c:365 rtw_get_stainfo23a()
 error: potential NULL dereference 'psta'.

Fixes: e280d71b("staging: rtl8723au: use list_for_each_entry*()")
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>