Fix minor issues found by coverity scan:
 - Do not try to estimate slice size if the run is empty to avoid div/0.
 - Fix assert() that is supposed to check slice->first_page_no, but
   checks last_page_no instead.
 - If the slice doesn't intersect with the run (i.e. slice->end is less
   than or equal to run->begin), leave its stats zeroed out.