Tighten asserts on ParallelWorkerNumber

The comment about ParallelWorkerNumbr in parallel.c says:

  In parallel workers, it will be set to a value >= 0 and < the number
  of workers before any user code is invoked; each parallel worker will
  get a different parallel worker number.

However asserts in various places collecting instrumentation allowed
(ParallelWorkerNumber == num_workers). That would be a bug, as the value
is used as index into an array with num_workers entries.

Fixed by adjusting the asserts accordingly. Backpatch to all supported
versions.

Discussion: https://postgr.es/m/5db067a1-2cdf-4afb-a577-a04f30b69167@vondra.me
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Backpatch-through: 14

src/backend/executor/nodeAgg.c

@@ -4386,7 +4386,7 @@ ExecEndAgg(AggState *node)
 	{
 		AggregateInstrumentation *si;
 
-		Assert(ParallelWorkerNumber <= node->shared_info->num_workers);
+		Assert(ParallelWorkerNumber < node->shared_info->num_workers);
 		si = &node->shared_info->sinstrument[ParallelWorkerNumber];
 		si->hash_batches_used = node->hash_batches_used;
 		si->hash_disk_used = node->hash_disk_used;

src/backend/executor/nodeIncrementalSort.c

@@ -103,7 +103,7 @@
 			if ((node)->shared_info && (node)->am_worker) \
 			{ \
 				Assert(IsParallelWorker()); \
-				Assert(ParallelWorkerNumber <= (node)->shared_info->num_workers); \
+				Assert(ParallelWorkerNumber < (node)->shared_info->num_workers); \
 				instrumentSortedGroup(&(node)->shared_info->sinfo[ParallelWorkerNumber].groupName##GroupInfo, \
 									  (node)->groupName##_state); \
 			} \

src/backend/executor/nodeMemoize.c

@@ -1109,7 +1109,7 @@ ExecEndMemoize(MemoizeState *node)
 		if (node->stats.mem_peak == 0)
 			node->stats.mem_peak = node->mem_used;
 
-		Assert(ParallelWorkerNumber <= node->shared_info->num_workers);
+		Assert(ParallelWorkerNumber < node->shared_info->num_workers);
 		si = &node->shared_info->sinstrument[ParallelWorkerNumber];
 		memcpy(si, &node->stats, sizeof(MemoizeInstrumentation));
 	}

src/backend/executor/nodeSort.c

@@ -134,7 +134,7 @@ ExecSort(PlanState *pstate)
 			TuplesortInstrumentation *si;
 
 			Assert(IsParallelWorker());
-			Assert(ParallelWorkerNumber <= node->shared_info->num_workers);
+			Assert(ParallelWorkerNumber < node->shared_info->num_workers);
 			si = &node->shared_info->sinstrument[ParallelWorkerNumber];
 			tuplesort_get_stats(tuplesortstate, si);
 		}