Add a libfuzzer for RtpHeaderParser.

webrtc/modules/rtp_rtcp/source/rtp_sender.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_sender.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
index 9b6b72ae1f10f0886fb4a154719ec4070425460a..4ee2524abc3153c8103fa9d0377e1e056d13b6f9 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
@@ -286,6 +286,7 @@ int32_t RTPSender::RegisterRtpHeaderExtension(RTPExtensionType type,
     case kRtpExtensionTransportSequenceNumber:
       return rtp_header_extension_map_.Register(type, id);
     case kRtpExtensionNone:
+    case kRtpExtensionNumberOfExtensions:
       LOG(LS_ERROR) << "Invalid RTP extension type for registration";
       return -1;
   }