Don't reset latch in ConditionVariablePrepareToSleep().

Don't reset latch in ConditionVariablePrepareToSleep().

It's not OK to do that without calling CHECK_FOR_INTERRUPTS().
Let the next wait loop deal with it, following the usual pattern.

One consequence of this bug was that a SIGTERM delivered in a very
narrow timing window could leave a parallel worker process waiting
forever for a condition variable that will never be signaled, after
an error was raised in other process.

The code is a bit different in the stable branches due to commit
1321509f, making problems less likely there.  No back-patch for now,
but we may finish up deciding to make a similar change after more
discussion.

Author: Thomas Munro
Reviewed-by: Shawn Debnath
Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/CA%2BhUKGJOm8zZHjVA8svoNT3tHY0XdqmaC_kHitmgXDQM49m1dA%40mail.gmail.com