fix out-of-bound read within LZ4_decompress_fast()

fix out-of-bound read within LZ4_decompress_fast()

and deprecate LZ4_decompress_fast(),
with deprecation warnings enabled by default.

Note that, as a consequence of the fix,
LZ4_decompress_fast is now __slower__ than LZ4_decompress_safe().
That's because, since it doesn't know the input buffer size,
it must progress more cautiously into the input buffer
to ensure to out-of-bound read.