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

webrtc/pc/webrtcsdp.cc

Index: webrtc/pc/webrtcsdp.cc
diff --git a/webrtc/pc/webrtcsdp.cc b/webrtc/pc/webrtcsdp.cc
index 11eefe5c3d5098f88ce8a884c5535d7dd14248cf..601e6437d6eb1cc24adb0f5914358f851f8c1347 100644
--- a/webrtc/pc/webrtcsdp.cc
+++ b/webrtc/pc/webrtcsdp.cc
@@ -2122,10 +2122,17 @@ static bool ParseMsidAttribute(const std::string& line,
     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;
 }