cfg80211: add two missing NULL pointer checks

These pointers can be NULL, the is_mesh() case isn't
ever hit in the current kernel, but cmp_ies() can be
hit under certain conditions.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.29, 2.6.30]
Signed-off-by: John W. Linville <linville@tuxdriver.com>