mkvparser,Block::Parse: remove incorrect assert
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

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm