Issue 747 (and others?): Avoid OOB read when parsing multiple long lines

Issue 747 (and others?):  Avoid OOB read when parsing multiple long lines

The mtree bidder needs to look several lines ahead
in the input.  It does this by extending the read-ahead
and parsing subsequent lines from the same growing buffer.
A bookkeeping error when extending the read-ahead would
sometimes lead it to significantly over-count the
size of the line being read.