Check collector has started in pg_wait_sampling_reset_profile.

The worker might have not started yet or it may never start,
because its registration was cancelled due to worker limit.

This commit adds a check for NULL value of pgws_collector_hdr->latch.
The previous usage in pg_wait_sampling.c has such a check, we
should do the same here.

Author: Valeriy Zainullin

pg_wait_sampling.c

@@ -819,6 +819,18 @@ pg_wait_sampling_reset_profile(PG_FUNCTION_ARGS)
 	pgws_collector_hdr->request = PROFILE_RESET;
 	LockRelease(&collectorTag, ExclusiveLock, false);
 
+        /* The collector may have not started yet (it'd be a
+         * race condition with background worker startup to
+         * use latch without a check here).
+         * Or worse, registration of the worker might have been
+         * denied, if there are too many workers already.
+         * This is why we cannot wait for its start with a while
+         * loop.
+         */
+	if (!pgws_collector_hdr->latch)
+		ereport(ERROR, (errcode(ERRCODE_INTERNAL_ERROR),
+						errmsg("pg_wait_sampling collector wasn't started")));
+
 	SetLatch(pgws_collector_hdr->latch);
 
 	LockRelease(&queueTag, ExclusiveLock, false);