Confusing step in `parse a vint`

[simonwuelker]

Related to #93.
Steps 5 and 6 in the algorithm are

5. Let index be 0. 
6. Let parsed number be sequence[index] & ~mask. 

This seems like a mistake, since the first four bytes of sequence have already been compared to static values at the beginning of the matches the signature for WebM algorithm (which is the only caller of parse a vint) and therefore can't contain data that we would need to parse.

Would I be correct in assuming that index should initially be equal to iter instead?