BusDesktopFile: Don't leak content if key=value appears before [Section]

BusDesktopFile has a strange convention in which the various parser
helper functions (parse_section_start(), etc.) free the parser on error.
However, this particular error case happens outside the helper functions
and so will leak.

Signed-off-by: Simon McVittie <smcv@collabora.com>

bus/desktop-file.c

@@ -689,6 +689,7 @@ bus_desktop_file_load (DBusString *filename,
 	{
            dbus_set_error(error, DBUS_ERROR_FAILED,
                           "invalid service file: key=value before [Section]");
+           parser_free (&parser);
            return NULL;
 	}
       else