Cherry-pick b273da8. rdar://problem/109364379

    [WebRTC] oob in RTCStatsCollector::ProduceTransportStats_n
    https://bugs.webkit.org/show_bug.cgi?id=256516
    rdar://109084778

    Reviewed by Eric Carlson.

    Cherry-pick libwebrtc 124d7c3fe5bdc79a355c9df02d07f25331631a68, which makes sure to check that the iterator is valid before using it.

    * Source/ThirdParty/libwebrtc/Source/webrtc/pc/rtc_stats_collector.cc:

    Canonical link: https://commits.webkit.org/259548.747@safari-7615-branch

Identifier: 245886.886@safari-7613.4.1.0-branch

Source/ThirdParty/libwebrtc/Source/webrtc/pc/rtc_stats_collector.cc

@@ -2001,16 +2001,17 @@ void RTCStatsCollector::ProduceTransportStats_n(
     // exist.
     const auto& certificate_stats_it =
         transport_cert_stats.find(transport_name);
+    std::string local_certificate_id, remote_certificate_id;
     RTC_DCHECK(certificate_stats_it != transport_cert_stats.cend());
-    std::string local_certificate_id;
-    if (certificate_stats_it->second.local) {
-      local_certificate_id = RTCCertificateIDFromFingerprint(
-          certificate_stats_it->second.local->fingerprint);
-    }
-    std::string remote_certificate_id;
-    if (certificate_stats_it->second.remote) {
-      remote_certificate_id = RTCCertificateIDFromFingerprint(
-          certificate_stats_it->second.remote->fingerprint);
+    if (certificate_stats_it != transport_cert_stats.cend()) {
+      if (certificate_stats_it->second.local) {
+        local_certificate_id = RTCCertificateIDFromFingerprint(
+            certificate_stats_it->second.local->fingerprint);
+      }
+      if (certificate_stats_it->second.remote) {
+        remote_certificate_id = RTCCertificateIDFromFingerprint(
+            certificate_stats_it->second.remote->fingerprint);
+      }
     }
 
     // There is one transport stats for each channel.