unitig_visitor: don't count the same graph store twice
unitig_visitor: don't count the same graph store twice

When a graph store has nothing more to produce, but other graph
stores still do, a graph store may be counted more than once.

This change fixes this bug.

Suggested-by: Élénie Godzaridis <34054f3e712362e45f66b09b447f670399d9bcd2@gmail.com>
Signed-off-by: Sébastien Boisvert <1a4d2e25eb5ba040b6ad5779a730a1a8a0ff9bc5@anl.gov>

GeneAssembly/biosal,sebhtml/biosal,sebhtml/biosal,GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal,sebhtml/biosal,GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal,GeneAssembly/biosal,sebhtml/biosal