Initialize variable to silence compiler warning.

After ff11e7f4b9 Tom's compiler warns about accessing a potentially
uninitialized rInfo. That's not actually possible, but it's
understandable the compiler would get this wrong. NULL initialize too.

Reported-By: Tom Lane
Discussion: https://postgr.es/m/11199.1551285318@sss.pgh.pa.us

src/backend/commands/trigger.c

@@ -4490,7 +4490,7 @@ afterTriggerInvokeEvents(AfterTriggerEventList *events,
 	AfterTriggerEventChunk *chunk;
 	MemoryContext per_tuple_context;
 	bool		local_estate = false;
-	ResultRelInfo *rInfo;
+	ResultRelInfo *rInfo = NULL;
 	Relation	rel = NULL;
 	TriggerDesc *trigdesc = NULL;
 	FmgrInfo   *finfo = NULL;