Block::Parse: replace pos asserts w/checks
Block::Parse: replace pos asserts w/checks

replace the common patterns assert(pos <= stop) / assert(pos == stop)
with error checks

BUG=23431751

Change-Id: Ia1265a639086c790a2ed542f34c2f438c153b036

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm,zofuthan/libwebm,felipebetancur/libwebm,felipebetancur/libwebm,zofuthan/libwebm