Avoid setting unused variables

The range partition handling in get_part_rule() seemed to have some
parts copypasted from the list handling, and subsequently ended up
looking at and setting variables never to be read. Remove the unused
code and also ensure to exit early in case we weren't called with
the correct data.
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>

src/backend/cdb/cdbpartition.c

@@ -4899,6 +4899,9 @@ get_part_rule(Relation rel,
 	char		relnamBuf[(NAMEDATALEN * 2)];
 	char	   *relname;
 
+	if (!pid)
+		return NULL;
+
 	snprintf(relnamBuf, sizeof(relnamBuf), "relation \"%s\"",
 			 RelationGetRelationName(rel));
 
@@ -4913,9 +4916,6 @@ get_part_rule(Relation rel,
 							  bExistError, bMustExist,
 							  pSearch, pNode, relname, NULL);
 
-	if (!pid)
-		return NULL;
-
 	if (pid->idtype == AT_AP_IDRule)
 	{
 		List	   *l1 = (List *) pid->partiddef;
@@ -4932,20 +4932,8 @@ get_part_rule(Relation rel,
 
 		pid2 = (AlterPartitionId *) lfirst(lc);
 
-		prule2 = get_part_rule1(rel,
-								pid2,
-								bExistError, bMustExist,
-								pSearch, pNode, pstrdup(prule2->relname), &pNode2);
-
-		pNode = pNode2;
-
-		if (!pNode)
-		{
-			if (prule2 && prule2->topRule && prule2->topRule->children)
-				pNode = prule2->topRule->children;
-		}
-
-		return prule2;
+		return get_part_rule1(rel, pid2, bExistError, bMustExist, pSearch,
+							  pNode, pstrdup(prule2->relname), &pNode2);
 	}
 
 	if (pid->idtype == AT_AP_IDList)