workaround for decompression failure on Mobile Safari (iOS 6.0, 6.1)

zlib.js occasionally uses `!== void 0` to detect out-of-bounds array access.

However, Mobile Safari bundled with iOS 6.0 and 6.1 sometimes incorrectly return 0 in such access (https://gist.github.com/kazuho/5981164 illustrates the bug), and decompression fails.

__This pull-request is incomplete__; I have only implemented workaround for `inflate_stream.js` (sorry for that), but I am anyways sending the request to notify others of the problem.

Thank you for publishing zlib.js.