FILS: Do not leave error value in left counter

If fils_decrypt_assoc() were to fail on the AP side, the previous
implementation could have continued through the response generation
using left = -1. That could have resulted in unexpected processing if
this value were to be used as the length of the remaining (unencrypted)
IEs. Fix this by not updating left in the failure case.

Fixes: 78815f3dde6e ("FILS: Decrypt Association Request elements and check Key-Auth (AP)")
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/ap/ieee802_11.c

@@ -3221,6 +3221,8 @@ static void handle_assoc(struct hostapd_data *hapd,
 	if (sta->auth_alg == WLAN_AUTH_FILS_SK ||
 	    sta->auth_alg == WLAN_AUTH_FILS_SK_PFS ||
 	    sta->auth_alg == WLAN_AUTH_FILS_PK) {
+		int res;
+
 		/* The end of the payload is encrypted. Need to decrypt it
 		 * before parsing. */
 
@@ -3230,13 +3232,14 @@ static void handle_assoc(struct hostapd_data *hapd,
 			goto fail;
 		}
 
-		left = fils_decrypt_assoc(sta->wpa_sm, sta->fils_session, mgmt,
-					  len, tmp, left);
-		if (left < 0) {
+		res = fils_decrypt_assoc(sta->wpa_sm, sta->fils_session, mgmt,
+					 len, tmp, left);
+		if (res < 0) {
 			resp = WLAN_STATUS_UNSPECIFIED_FAILURE;
 			goto fail;
 		}
 		pos = tmp;
+		left = res;
 	}
 #endif /* CONFIG_FILS */