Cope with initialising huffman codecs with zero symbols.
1) Force array bounds checking on reference sequences before trying to
compute their MD5sum. This is needed when the slice coordinates are
invalid.

2) Improved the ambiguous TLEN case where both start and end are at the
same point.

Ie

-----------> first
-----------> last

or

-----------> first
<----------- last

It's valid at this stage to set both to be +ve as technically each is
both the leftmost and rightmost. However we now disambiguate this case
by the first/last bit flags instead.