Initialize fast planner restriction context (#4349)

We initialize fast planner restriction context so that code paths that
rely on this being not NULL will operate without a problem.

src/backend/distributed/planner/relation_restriction_equivalence.c

@@ -1808,6 +1808,8 @@ FilterPlannerRestrictionForQuery(PlannerRestrictionContext *plannerRestrictionCo
 	/* allocate the filtered planner restriction context and set all the fields */
 	PlannerRestrictionContext *filteredPlannerRestrictionContext = palloc0(
 		sizeof(PlannerRestrictionContext));
+	filteredPlannerRestrictionContext->fastPathRestrictionContext =
+		palloc0(sizeof(FastPathRestrictionContext));
 
 	filteredPlannerRestrictionContext->memoryContext =
 		plannerRestrictionContext->memoryContext;