dvd-sector-copy.c: Calculate the clear-area size consistently.

dvd-sector-copy.c: Calculate the clear-area size consistently.

This should be determined as a (possibly fractional) multiple of the
distance from the start of the bad region to the current position.
Alas, the code for the bounding stage incorrectly used the distance from
the previous position to the new one.  The inconsistency between the
bounding and refinement stages of the recovery algorithm could lead to
failed invariants and a nonsense result.