Fix #7242, CALL(@0) crash backend

When executing a prepared CALL, which is not pure SQL but available
with some drivers like npgsql and jpgdbc, Citus entered a code path where
a plan is not defined, while trying to increase its cost.
Thus SIG11 when plan is a NULL pointer.

Fix by just increasing plan cost when plan is not null.

However, it is a bit suspicious to get here with a NULL plan and maybe a
better change will be to not call ShardPlacementForFunctionColocatedWithDistTable()
with a NULL plan at all (in call.c:134)

src/backend/distributed/planner/distributed_planner.c

@@ -702,6 +702,7 @@ DissuadePlannerFromUsingPlan(PlannedStmt *plan)
 	 * Arbitrarily high cost, but low enough that it can be added up
 	 * without overflowing by choose_custom_plan().
 	 */
+	Assert(plan != NULL);
 	plan->planTree->total_cost = FLT_MAX / 100000000;
 }
 

src/backend/distributed/planner/function_call_delegation.c

@@ -526,7 +526,10 @@ ShardPlacementForFunctionColocatedWithDistTable(DistObjectCacheEntry *procedure,
 		if (partitionParam->paramkind == PARAM_EXTERN)
 		{
 			/* Don't log a message, we should end up here again without a parameter */
-			DissuadePlannerFromUsingPlan(plan);
+			if (plan)
+			{
+				DissuadePlannerFromUsingPlan(plan);
+			}
 			return NULL;
 		}
 	}