mkvparser,Block::Parse: remove incorrect assert

this path has been error checked since:
986b64b mkvparser: Add error checking in Block::Parse.

BUG=webm:1405

Change-Id: I1c17c05076455f25ab888555d71d7b27c032e9dd
diff --git a/mkvparser/mkvparser.cc b/mkvparser/mkvparser.cc
index 80814e0..6e1c26e 100644
--- a/mkvparser/mkvparser.cc
+++ b/mkvparser/mkvparser.cc
@@ -7936,7 +7936,6 @@
     pf = m_frames;
     while (pf != pf_end) {
       Frame& f = *pf++;
-      assert((pos + f.len) <= stop);
       if ((pos + f.len) > stop)
         return E_FILE_FORMAT_INVALID;