gio: Update GMemoryOutputStream length after truncate

For GMemoryOutputStream, update valid_len when truncating so that
g_memory_output_stream_get_data_size () returns the correct result.

https://bugzilla.gnome.org/show_bug.cgi?id=720080

gio/gmemoryoutputstream.c

@@ -828,6 +828,8 @@ g_memory_output_stream_truncate (GSeekable     *seekable,
   if (!array_resize (ostream, offset, FALSE, error))
     return FALSE;
 
+  ostream->priv->valid_len = offset;
+
   return TRUE;
 }