wifi: mac80211: check defragmentation succeeded

We need to check that cfg80211_defragment_element()
didn't return an error, since it can fail due to bad
input, and we didn't catch that before.

Fixes: 8eb8dd2f ("wifi: mac80211: Support link removal using Reconfiguration ML element")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20231211085121.8595a6b67fc0.I1225edd8f98355e007f96502e358e476c7971d8c@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -5782,7 +5782,7 @@ static void ieee80211_ml_reconfiguration(struct ieee80211_sub_if_data *sdata,
 {
 	const struct ieee80211_multi_link_elem *ml;
 	const struct element *sub;
-	size_t ml_len;
+	ssize_t ml_len;
 	unsigned long removed_links = 0;
 	u16 link_removal_timeout[IEEE80211_MLD_MAX_NUM_LINKS] = {};
 	u8 link_id;
@@ -5798,6 +5798,8 @@ static void ieee80211_ml_reconfiguration(struct ieee80211_sub_if_data *sdata,
 					     elems->scratch + elems->scratch_len -
 					     elems->scratch_pos,
 					     WLAN_EID_FRAGMENT);
+	if (ml_len < 0)
+		return;
 
 	elems->ml_reconf = (const void *)elems->scratch_pos;
 	elems->ml_reconf_len = ml_len;