Fix bug where renaming a tablespace was leading to an error

When renaming a tablespace, it was renamed first, then a comment was set
on its old name in the same xact, leading to an error and rollback.

classes/database/Postgres.php

@@ -7031,10 +7031,12 @@ function alterTablespace($spcname, $name, $owner, $comment='') {
 		if ($name != $spcname) {
 			$sql = "ALTER TABLESPACE \"{$spcname}\" RENAME TO \"{$name}\"";
 			$status = $this->execute($sql);
-		if ($status != 0) {
-			$this->rollbackTransaction();
-			return -3;
-		}
+			if ($status != 0) {
+				$this->rollbackTransaction();
+				return -3;
+			}
+
+			$spcname = $name;
 		}
 
 		// Set comment if it has changed