webmdec: Handle codec id being NULL.

WebM files could have CodecId missing in the track headers. Treat those files as
unknown input file type in vpxdec.

Fixes issue #1064.

Change-Id: I6c3bb7b4bd3a4f5c244312482a5996f8b68db3f3

webmdec.cc

@@ -94,7 +94,7 @@ int file_is_webm(struct WebmInputContext *webm_ctx,
     }
   }
 
-  if (video_track == NULL) {
+  if (video_track == NULL || video_track->GetCodecId() == NULL) {
     rewind_and_reset(webm_ctx, vpx_ctx);
     return 0;
   }