plug-ins: fix using wrong offset for group layer masks in psd export.

Exporting to psd could result in very large files which often failed to
load in both GIMP and PS when a group layer with a layer mask
was present.

Reported on our IRC channel with a sample xcf which made it
possible to figure out the problem.

plug-ins/file-psd/psd-save.c

@@ -1538,7 +1538,9 @@ write_pixel_data (FILE         *fd,
 
       if (ChanLenPosition)    /* Update total compressed length */
         {
-          fseek (fd, ChanLenPosition[bytes], SEEK_SET); /*+bytes OR SOMETHING*/
+          /* Mask follows other components so use that as offset. */
+          fseek (fd, ChanLenPosition[components], SEEK_SET);
+
           write_gint32 (fd, len, "channel data length");
           IFDBG printf ("\t\tUpdating data len to %d, at %ld\n", len, ftell(fd));
         }