Update maximum chromosome length for PNEXT/TLEN

These fields were accidentally omitted from commit 28c54ab,
which updated the maximum chromosome length for the POS field.

Depending on whether TLEN counts fence posts or gaps between posts,
this may be problematic when POS=0 and PNEXT=2^31-1.  This case should
perhaps be considered further if we clarify TLEN's definition.