Avoid theoretical infinite loop loading relcache partition key.

Amit Langote, per report from 甄明洋

Discussion: http://postgr.es/m/57bd1e1.1886.15bd7b79cee.Coremail.18612389267@yeah.net

src/backend/utils/cache/relcache.c

@@ -3858,13 +3858,20 @@ RelationCacheInitializePhase3(void)
 		}
 
 		/*
-		 * Reload partition key and descriptor for a partitioned table.
+		 * Reload the partition key and descriptor for a partitioned table.
 		 */
-		if (relation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		if (relation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE &&
+			relation->rd_partkey == NULL)
 		{
 			RelationBuildPartitionKey(relation);
 			Assert(relation->rd_partkey != NULL);
 
+			restart = true;
+		}
+
+		if (relation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE &&
+			relation->rd_partdesc == NULL)
+		{
 			RelationBuildPartitionDesc(relation);
 			Assert(relation->rd_partdesc != NULL);