Fix indexing integer overflow for 15-bit window

[sameer]

Partly addresses #86

A 15-bit window with a filled input buffer can result in an integer overflow in do_indexing. This PR addresses the problem by making the end bound inclusive.

One solution I came up with to actually get indexing to work for a 15-bit window was to make the index an unsigned 16-bit integer array with 0xFFFF representing end-of-list.

[Megatokio]

this does not work. this replaces a loop test of "i<0" which never runs with "i<=65535" which never finishs. If you use int32 then this part works. But there are other places where W15 does not work properly. I made some tests with and without HEATSHRINK_USE_INDEX and compression with W15 is almost nothing. i didn't test whether the output is correct at all.

The README.MD should state that heatshrink only works for W4 to W14.
And possibly include a test for this.

kio !