Fix SDP parsing crash due to missing track ID in "a=msid".

webrtc/pc/webrtcsdp.cc

 /*
  *  Copyright 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 2104 matching lines @@
   // msid-value = msid-id [ SP msid-appdata ]
   // msid-id = 1*64token-char ; see RFC 4566
   // msid-appdata = 1*64token-char  ; see RFC 4566
   std::string field1;
   if (!rtc::tokenize_first(line.substr(kLinePrefixLength), kSdpDelimiterSpace,
                            &field1, track_id)) {
     const size_t expected_fields = 2;
     return ParseFailedExpectFieldNum(line, expected_fields, error);
   }
 
+  if (track_id->empty()) {
+    return ParseFailed(line, "Missing track ID in msid attribute.", error);
+  }
+
   // msid:<msid-id>
   if (!GetValue(field1, kAttributeMsid, stream_id, error)) {
     return false;
   }
+  if (stream_id->empty()) {
+    return ParseFailed(line, "Missing stream ID in msid attribute.", error);
+  }
   return true;
 }
 
 // RFC 3551
 //  PT   encoding    media type  clock rate   channels
 //                      name                    (Hz)
 //  0    PCMU        A            8,000       1
 //  1    reserved    A
 //  2    reserved    A
 //  3    GSM         A            8,000       1
@@ skipping 1052 matching lines @@
     UpdateCodec<AudioContentDescription, cricket::AudioCodec>(
         media_desc, payload_type, feedback_param);
   } else if (media_type == cricket::MEDIA_TYPE_VIDEO) {
     UpdateCodec<VideoContentDescription, cricket::VideoCodec>(
         media_desc, payload_type, feedback_param);
   }
   return true;
 }
 
 }  // namespace webrtc