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.

in the projects i have seen so far, ensure had the meaning of getOrCreate, which in this context would be: if its there ok, otherwise load it.
I personally would prefer "check" or "verify" as name, but thats just bikeshedding :)