mkvparser: validate results in EBMLHeader::Parse.

Return an error when DocType, DocTypeReadVersion, DocTypeVersion
EBMLMaxIDLength or EBMLMaxSizeLength are invalid or cannot be
handled by mkvparser.

Update samples to respect the return value from EBMLHeader::Parse.

BUG=https://code.google.com/p/webm/issues/detail?id=1057

Change-Id: I9337f13c1d5fa366b1101e48fe6bc46eb5b2ee97

vttdemux.cc

@@ -345,7 +345,7 @@ bool vttdemux::ParseHeader(mkvparser::IMkvReader* reader, mkvpos_t* pos) {
     return false;
   }
 
-  if (strcmp(h.m_docType, "webm") != 0) {
+  if (h.m_docType == NULL || strcmp(h.m_docType, "webm") != 0) {
     printf("bad doctype\n");
     return false;
   }