Fix pgbench options -C and -R together

Fix pgbench options -C and -R together

The bug is that prior to --rate doCustom was always disconnect/reconnect
without exiting, but with rate it returns if it has to wait. However threadRun
test whether there is a connection before recalling doCustom, so it was never
called.

Bug is not existed in head branch because of refactoring at
12788ae49e1933f463bc59a6efe46c4a01701b76, patch only 9.6

Author: Fabien Coelho
Reviewed-by: me

https://commitfest.postgresql.org/13/970/