ensure conformance with custom LZ4_DISTANCE_MAX

ensure conformance with custom LZ4_DISTANCE_MAX

It's now possible to select a custom LZ4_DISTANCE_MAX at compile time,
provided it's <= 65535.

However, in some cases (when compressing in byU16 mode),
the new distance wasn't respected,
as it used to implied that it was necessarily within range.

Added a distance check for this case.
Also : added a new TravisCI test which ensures that
custom LZ4_DISTANCE_MAX compiles correctly
and compresses correctly (relying on `assert()` to find outsized offsets).