Fix possible lockup in pgbench with -R.

pgbench would sometimes get stuck waiting forever after its last
client thread terminated, due to failing to check for there being
nothing more to wait for.

Bug introduced during refactoring in v10 (I didn't bother to try to
assign blame to a specific commit).  It's already repaired in
HEAD/v12 thanks to commit 3bac77c4, but v10 and v11 need this fix.

Fabien Coelho, per report from Tomas Vondra; reviewed by
Yoshikazu Imai

Discussion: https://postgr.es/m/cc5d76c1-6144-bbed-ad1b-961d13d88f3b@2ndquadrant.com