Adjust the number of events given to WaitEventSetWait() so that it
doesn't exceed the maximum number of events in the WaitEventSet given
to that function (set->nevents_space) in hopes of making the buildfarm
green.

Per valgrind failure report from Tom Lane and the buildfarm.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/3411577.1617289776%40sss.pgh.pa.us