cfg80211: fix NULL dereference in IBSS SIOCGIWAP

This patch avoids memcpy from wdev->wext.ibss.bssid if it is NULL.
This could happen if we SIOCGIWAP before SIOCSIWAP.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>