Fix optimization for skipping searches for parallel-query hazards.

Fix thinko in commit da1c91631: even if the original query was free of
parallel hazards, we might introduce such a hazard by adding PARAM_EXEC
Param nodes.  Adjust is_parallel_safe() so that it will scan the given
expression whenever any such nodes have been created.  Per report from
Andreas Seltenreich.

Discussion: <878tse6yvf.fsf@credativ.de>

src/backend/optimizer/util/clauses.c

@@ -1150,8 +1150,14 @@ is_parallel_safe(PlannerInfo *root, Node *node)
 {
 	max_parallel_hazard_context context;
 
-	/* If max_parallel_hazard found nothing unsafe, we don't need to look */
-	if (root->glob->maxParallelHazard == PROPARALLEL_SAFE)
+	/*
+	 * Even if the original querytree contained nothing unsafe, we need to
+	 * search the expression if we have generated any PARAM_EXEC Params while
+	 * planning, because those are parallel-restricted and there might be one
+	 * in this expression.  But otherwise we don't need to look.
+	 */
+	if (root->glob->maxParallelHazard == PROPARALLEL_SAFE &&
+		root->glob->nParamExec == 0)
 		return true;
 	/* Else use max_parallel_hazard's search logic, but stop on RESTRICTED */
 	context.max_hazard = PROPARALLEL_SAFE;