Guard against core dump from uninitialized subplan.

If the planner erroneously puts a non-parallel-safe SubPlan into
a parallelized portion of the query tree, nodeSubplan.c will fail
in the worker processes because it finds a null in es_subplanstates,
which it's unable to cope with.  It seems worth a test-and-elog to
make that an error case rather than a core dump case.

This probably should have been included in commit 16ebab68, which
was responsible for allowing nulls to appear in es_subplanstates
to begin with.  So, back-patch to v10 where that came in.

Discussion: https://postgr.es/m/924226.1604422326@sss.pgh.pa.us

src/backend/executor/nodeSubplan.c

@@ -797,7 +797,15 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 	sstate->planstate = (PlanState *) list_nth(estate->es_subplanstates,
 											   subplan->plan_id - 1);
 
-	/* ... and to its parent's state */
+	/*
+	 * This check can fail if the planner mistakenly puts a parallel-unsafe
+	 * subplan into a parallelized subquery; see ExecSerializePlan.
+	 */
+	if (sstate->planstate == NULL)
+		elog(ERROR, "subplan \"%s\" was not initialized",
+			 subplan->plan_name);
+
+	/* Link to parent's state, too */
 	sstate->parent = parent;
 
 	/* Initialize subexpressions */