extent scan: don't divide by zero if there were no loops

extent scan: don't divide by zero if there were no loops

Commit 183b6a5361e040d7cc70c3b3391f43e0d126bc33 ("extent scan: refactor
BeesCrawl, BeesScanMode*") moved some statistics calculations out of
the loop in `find_next_extent`, but did not ensure that the statistics
would not be calculated if the loop had not executed any iterations.

In rare instances, the function returns without entering the loop at all,
which results in divide by zero.  Add a check just before doing that.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>