wifi: mac80211: init cnt before accessing elem in ieee80211_copy_mbssid_beacon

wifi: mac80211: init cnt before accessing elem in ieee80211_copy_mbssid_beacon

With the new __counted_by annocation in cfg80211_mbssid_elems,
the "cnt" struct member must be set before accessing the "elem"
array. Failing to do so will trigger a runtime warning when enabling
CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE.

Fixes: c14679d7005a ("wifi: cfg80211: Annotate struct cfg80211_mbssid_elems with __counted_by")
Signed-off-by: Haoyu Li <lihaoyu499@gmail.com>
Link: https://patch.msgid.link/20241123172500.311853-1-lihaoyu499@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>