Mistake in ab0dfc96; progress reporting would have wrapped around
for indexes created with more than 2^31 tuples.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wz=WbNxc5ob5NJ9yqo2RMJ0q4HXDS30GVCobeCvC9A1L9A@mail.gmail.com