Silence compiler warnings

In GetCachedPlan(), initialize 'plan' to silence a compiler warning, but
also add an Assert() to make sure we don't ever actually fall through
with 'plan' still being set to NULL, since we are about to dereference
it.

Back-patch back to 9.2.

Author: Stephen Frost
Discussion: https://postgr.es/m/20161129152102.GR13284%40tamriel.snowman.net

src/backend/utils/cache/plancache.c

@@ -1125,7 +1125,7 @@ CachedPlan *
 GetCachedPlan(CachedPlanSource *plansource, ParamListInfo boundParams,
 			  bool useResOwner)
 {
-	CachedPlan *plan;
+	CachedPlan *plan = NULL;
 	List	   *qlist;
 	bool		customplan;
 
@@ -1207,6 +1207,8 @@ GetCachedPlan(CachedPlanSource *plansource, ParamListInfo boundParams,
 		}
 	}
 
+	Assert(plan != NULL);
+
 	/* Flag the plan as in use by caller */
 	if (useResOwner)
 		ResourceOwnerEnlargePlanCacheRefs(CurrentResourceOwner);