compression/huffman: avoid semi-defined behaviour in decompress

compression/huffman: avoid semi-defined behaviour in decompress

We had

               output[output_pos - distance];

where output_pos and distance are size_t and distance can be greater
than output_pos (because it refers to a place in the previous block).

The underflow is defined, leading to a big number, and when
sizeof(size_t) == sizeof(*uint8_t) the subsequent overflow works as
expected. But if size_t is smaller than a pointer, bad things will
happen.

This was found by OSSFuzz with
'UBSAN_OPTIONS=print_stacktrace=1:silence_unsigned_overflow=1'.

Credit to OSSFuzz.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>