fp->field_length was unsigned and therefore the negative
condition around it.

Backport of cc182aca fixes it, however to correct the
consistent use of types pcf->Length needs to be unsigned
too.

At one point pcf->Precision is assigned from pcf->Length so
that's also unsigned.

GetTypeSize is assigned to length and has a length argument.
A -1 default value seemed dangerious to case, so at least 0
should assert if every hit.