[Merge-110] [Stats] Handle the case of missing certificates.

Certificates being missing is a sign of a bug (e.g. webrtc:14844, to be
fixed separately) which is why we have a DCHECK. But this DCHECK does
not protect against accessing the invalid iterator if it is a release
build. This CL makes that safe.

# Mobile bots not running properly
NOTRY=True

(cherry picked from commit 124d7c3)

Bug: chromium:1408392
Change-Id: I97a82786028e41c58ef8ef15002c3f959bbec7f1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291109
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Original-Commit-Position: refs/heads/main@{#39159}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291380
Cr-Commit-Position: refs/branch-heads/5481@{#2}
Cr-Branched-From: 2e1a9a4-refs/heads/main@{#38901}

pc/rtc_stats_collector.cc

@@ -2192,16 +2192,17 @@ 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.