Faster chunkedStream_getByte()

Even though #4966 accelerated `ensureRange()` a lot, it is faster to avoid the check altogether. If the byte array is non-zero at the requested position, the data must have been loaded already, so we can skip the range check.

This speeds up some PDFs considerably, for example, page 11 of the [tracemonkey](http://cdn.mozilla.net/pdfjs/tracemonkey.pdf#pdfBug=Stats&page=11&zoom=50) paper now loads 20-30% faster.

> maybe we could rename ensureByte? (and then also fix the name of the anon function which is right now ensureRange)

This does not seem relevant to the current PR.
(Furthermore, I don't understand what's wrong (or unclear) with the currently used names. E.g. `ensureByte` ensures that the byte we're trying to get actually exists, so that name looks fine to me.)