iwmc3200wifi: fix NULL pointer dereference in pmkid update

When handling IWM_CMD_PMKID_FLUSH command, the bssid and
pmkid in pmksa are all NULL. Check it before memcpy.

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