mac80211_hwsim: Add missing check for HWSIM_ATTR_SIGNAL

A wmediumd that does not send this attribute causes a NULL pointer
dereference, as the attribute is accessed even if it does not exist.

The attribute was required but never checked ever since userspace frame
forwarding has been introduced. The issue gets more problematic once we
allow wmediumd registration from user namespaces.

Cc: stable@vger.kernel.org
Fixes: 7882513b

 ("mac80211_hwsim driver support userspace frame tx/rx")
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>