rt-migrate-test: exit early if nr_runs is non-positive

Program will crash if nr_runs is 0 due to dividing by it in
print_results(). Let's exit early instead.

Fixes: http://bugs.debian.org/716237
Signed-off-by: Boris Egorov <egorov@linux.com>
Signed-off-by: Clark Williams <williams@redhat.com>