[PATCH] hostap: Fix ap_add_sta() return value verification

The Coverity checker (CID: 273) spotted this inconsequent NULL checking
(unconditionally dereferencing directly after checking for NULL
isn't a good idea). Return immediately to avoid this.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Jouni Malinen <jkmaline@cc.hut.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>