cfg80211: check S1G beacon compat element length

We need to check the length of this element so that we don't
access data beyond its end. Fix that.

Fixes: 9eaffe50 ("cfg80211: convert S1G beacon to scan results")
Link: https://lore.kernel.org/r/20210408142826.f6f4525012de.I9fdeff0afdc683a6024e5ea49d2daa3cd2459d11@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>