Return nullptr from RTCCertificate::FromPEM on failure.

webrtc/base/rtccertificate_unittest.cc

Index: webrtc/base/rtccertificate_unittest.cc
diff --git a/webrtc/base/rtccertificate_unittest.cc b/webrtc/base/rtccertificate_unittest.cc
index f5df7f1130be8c143ccb2915177c4e5107e94aff..b318717790916f79e1afdda37cc63fcc39b591ba 100644
--- a/webrtc/base/rtccertificate_unittest.cc
+++ b/webrtc/base/rtccertificate_unittest.cc
@@ -137,4 +137,10 @@ TEST_F(RTCCertificateTest, CloneWithPEMSerialization) {
   EXPECT_EQ(orig->Expires(), clone->Expires());
 }
 
+TEST_F(RTCCertificateTest, FromPEMWithInvalidPEM) {
+  RTCCertificatePEM pem("not a valid PEM", "not a valid PEM");
+  scoped_refptr<RTCCertificate> certificate = RTCCertificate::FromPEM(pem);
+  EXPECT_FALSE(certificate);
+}
+
 }  // namespace rtc