unitig_walker: fix a bug in completion verification

A walker may count a given graph store more than once.

To avoid that problem, reset the counter when the index
is rewinded back to 0.

This solution was suggested by Élénie Godzaridis.

Suggested-by: Élénie Godzaridis <telomerika@gmail.com>
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>