Adjust the order of the prechecks in pgrowlocks()

4b8266415 added a precheck to pgrowlocks() to ensure the given object's
pg_class.relam is HEAP_TABLE_AM_OID, however, that check was put before
another check which was checking if the given object was a partitioned
table.  Since the pg_class.relam is always InvalidOid for partitioned
tables, if pgrowlocks() was called passing a partitioned table, then the
"only heap AM is supported" error would be raised instead of the intended
error about the given object being a partitioned table.

Here we simply move the pg_class.relam check to after the check that
verifies that we are in fact working with a normal (non-partitioned)
table.

Reported-by: jian he
Discussion: https://postgr.es/m/CACJufxFaSp_WguFCf0X98951zFVX+dXFnF1mxAb-G3g1HiHOow@mail.gmail.com
Backpatch-through: 12, where 4b8266415 was introduced.

contrib/pgrowlocks/pgrowlocks.c

@@ -81,10 +81,6 @@ pgrowlocks(PG_FUNCTION_ARGS)
 	relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
 	rel = relation_openrv(relrv, AccessShareLock);
 
-	if (rel->rd_rel->relam != HEAP_TABLE_AM_OID)
-		ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						errmsg("only heap AM is supported")));
-
 	if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
@@ -96,6 +92,10 @@ pgrowlocks(PG_FUNCTION_ARGS)
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 				 errmsg("\"%s\" is not a table",
 						RelationGetRelationName(rel))));
+	else if (rel->rd_rel->relam != HEAP_TABLE_AM_OID)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("only heap AM is supported")));
 
 	/*
 	 * check permissions: must have SELECT on table or be in