[gthumb] check if image is null before using it

[Paolo Bacchilega <paobac src gnome org>]

commit 1571cc4b961d9355fb90ef5934a3049b7062e597
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sun Sep 19 17:41:42 2021 +0200

    check if image is null before using it
    
    Fixes https://gitlab.gnome.org/GNOME/gthumb/-/issues/189

 gthumb/gth-image-viewer.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gthumb/gth-image-viewer.c b/gthumb/gth-image-viewer.c
index d8e8aad5..1c60022a 100644
--- a/gthumb/gth-image-viewer.c
+++ b/gthumb/gth-image-viewer.c
@@ -1618,6 +1618,8 @@ _gth_image_viewer_set_original_size (GthImageViewer *self,
        int              image_height;
 
        image = gth_image_viewer_get_current_image (self);
+       if (image == NULL)
+               return;
        image_width = cairo_image_surface_get_width (image);
        image_height = cairo_image_surface_get_height (image);