Return nullptr from RTCCertificate::FromPEM on failure.

BUG=webrtc:6488

Review-Url: https://codereview.webrtc.org/2424093002
Cr-Commit-Position: refs/heads/master@{#14710}
diff --git a/webrtc/base/rtccertificate.cc b/webrtc/base/rtccertificate.cc
index 574bf75..3b3b8c9 100644
--- a/webrtc/base/rtccertificate.cc
+++ b/webrtc/base/rtccertificate.cc
@@ -54,6 +54,8 @@
     const RTCCertificatePEM& pem) {
   std::unique_ptr<SSLIdentity> identity(SSLIdentity::FromPEMStrings(
       pem.private_key(), pem.certificate()));
+  if (!identity)
+    return nullptr;
   return new RefCountedObject<RTCCertificate>(identity.release());
 }