webm_info: check vp9 ParseUncompressedHeader return

avoids potential side effects

BUG=webm:1416,webm:1417

Change-Id: I4acd4c9f86be20fa3c06af6c87c1ce538a50c84c
diff --git a/webm_info.cc b/webm_info.cc
index 493b66b..1ce6590 100644
--- a/webm_info.cc
+++ b/webm_info.cc
@@ -716,7 +716,8 @@
       return;
     }
     parser->SetFrame(data, frame_length);
-    parser->ParseUncompressedHeader();
+    if (!parser->ParseUncompressedHeader())
+      return;
     level_stats->AddFrame(*parser, time_ns);
 
     // const int frame_marker = (data[0] >> 6) & 0x3;