BUG/MINOR: sample: Fix a possible underflow on be2hex for large chunk size

BUG/MINOR: sample: Fix a possible underflow on be2hex for large chunk size

For a chunk size exactly equal to bufsize/2, the "max_size" variable could
underflow if a separator is provided. Indeed, "max_size" is first set to
(trash->size - 2 * chunk_size), so to 0. And on the first iteration, the
separator length is removed, making it to underflow.

On 3.3 and lower it is especially an issue with samples larger than
bufsize/2 because data could be written ouside of the buffer. On 3.4 and
3.5, it is not an issue because we fail to retrieve a chunk.

This should be backported to all supported versions.