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.

If the byte at position `pos` has already been loaded, then it is correct to return `0` at `return byte`.
However, if `this.bytes[pos]` has not been loaded yet, then `this.ensureByte(pos)` will throw an exception and no value will be returned at all. I admit that this is not so obvious from the code.