[gimp] plug-ins: fix creating multiple preview display for JPEG export.

[Jehan <jehanp src gnome org>]

commit 9b033f8996e0e6be652348b0af4b640e93516685
Author: Jehan <jehan girinstud io>
Date:   Wed May 12 22:43:49 2021 +0200

    plug-ins: fix creating multiple preview display for JPEG export.
    
    Commit aba721ae68 fixed its intended bug but brought a new: each time
    the preview was updated, a new display was created. This fixes this new
    bug. Hopefully it's good now!

 plug-ins/file-jpeg/jpeg-save.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/plug-ins/file-jpeg/jpeg-save.c b/plug-ins/file-jpeg/jpeg-save.c
index 0ee08ff9bf..713031483e 100644
--- a/plug-ins/file-jpeg/jpeg-save.c
+++ b/plug-ins/file-jpeg/jpeg-save.c
@@ -743,7 +743,7 @@ make_preview (GimpProcedureConfig *config)
 
       g_object_unref (file);
 
-      if (separate_display)
+      if (separate_display && ! display)
         display = gimp_display_new (preview_image);
     }
   else