Avoid information leak in lzs_decompress() for invalid offset
Avoid information leak in lzs_decompress() for invalid offset

Encoded offset could be before start of valid data. This is invalid,
either by faulty encoder, or corrupted data, or maliciously created.
Avoid reading data before the start of the input buffer. Just output
zeros instead.

cmcqueen/lzs-compression,cmcqueen/lzs-compression,cmcqueen/lzs-compression