dvd-sector-copy.c (main): Use `last' to track final event for fixup.

dvd-sector-copy.c (main): Use `last' to track final event for fixup.

The `start' variable was doing double-duty, resulting in a possibly
erroneous value if the last event is `EV_WRITE', and a possibly
erroneous count of sectors.  Use `last' instead, because nothing else is
using this.