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: c14679d ("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>

Author: learjet5

net/mac80211/cfg.c

@@ -1088,13 +1088,13 @@ ieee80211_copy_mbssid_beacon(u8 *pos, struct cfg80211_mbssid_elems *dst,
 {
 	int i, offset = 0;
 
+	dst->cnt = src->cnt;
 	for (i = 0; i < src->cnt; i++) {
 		memcpy(pos + offset, src->elem[i].data, src->elem[i].len);
 		dst->elem[i].len = src->elem[i].len;
 		dst->elem[i].data = pos + offset;
 		offset += dst->elem[i].len;
 	}
-	dst->cnt = src->cnt;
 
 	return offset;
 }