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.

@fkaelberer Perhaps it would be a good idea to add a comment about `ensureByte` throwing an exception if the byte isn't loaded, to avoid confusion.

@samlh `ensureByte` doesn't modify this `this.bytes` in any way, please see: https://github.com/mozilla/pdf.js/blob/master/src/core/chunked_stream.js#L94.