[gimp] app: in GimpCanvasBufferPreview, take strong ref on buffer

[Ell <ell src gnome org>]

commit ea864e2d5c095a04317f0b70b240111dd240d044
Author: Ell <ell_se yahoo com>
Date:   Mon Feb 3 20:01:03 2020 +0200

    app: in GimpCanvasBufferPreview, take strong ref on buffer
    
    In GimpCanvasBufferPreview, take a strong reference on the preview
    buffer, fixing a segfault in the foreground-select tool when
    switching engines while the grayscale preview is active.

 app/display/gimpcanvasbufferpreview.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)
---
diff --git a/app/display/gimpcanvasbufferpreview.c b/app/display/gimpcanvasbufferpreview.c
index 26b1171787..858bab634b 100644
--- a/app/display/gimpcanvasbufferpreview.c
+++ b/app/display/gimpcanvasbufferpreview.c
@@ -58,6 +58,7 @@ struct _GimpCanvasBufferPreviewPrivate
 
 /*  local function prototypes  */
 
+static void             gimp_canvas_buffer_preview_dispose        (GObject               *object);
 static void             gimp_canvas_buffer_preview_set_property   (GObject               *object,
                                                                    guint                  property_id,
                                                                    const GValue          *value,
@@ -86,6 +87,7 @@ gimp_canvas_buffer_preview_class_init (GimpCanvasBufferPreviewClass *klass)
   GObjectClass        *object_class = G_OBJECT_CLASS (klass);
   GimpCanvasItemClass *item_class   = GIMP_CANVAS_ITEM_CLASS (klass);
 
+  object_class->dispose      = gimp_canvas_buffer_preview_dispose;
   object_class->set_property = gimp_canvas_buffer_preview_set_property;
   object_class->get_property = gimp_canvas_buffer_preview_get_property;
 
@@ -104,6 +106,16 @@ gimp_canvas_buffer_preview_init (GimpCanvasBufferPreview *transform_preview)
 {
 }
 
+static void
+gimp_canvas_buffer_preview_dispose (GObject *object)
+{
+  GimpCanvasBufferPreviewPrivate *private = GET_PRIVATE (object);
+
+  g_clear_object (&private->buffer);
+
+  G_OBJECT_CLASS (parent_class)->dispose (object);
+}
+
 static void
 gimp_canvas_buffer_preview_set_property (GObject      *object,
                                          guint         property_id,
@@ -115,7 +127,7 @@ gimp_canvas_buffer_preview_set_property (GObject      *object,
   switch (property_id)
     {
     case PROP_BUFFER:
-      private->buffer = g_value_get_object (value); /* don't ref */
+      g_set_object (&private->buffer, g_value_get_object (value));
       break;
 
     default: