Ensure valid thumb->zoom_100

In a few cornercases the calculation of thumb->zoom_100 fails. We can assume this
to be valid only if we have valid thumb->img_width > 0. (same fore height)

As we keep a thumbs zoom_100 for it's lifetime we don't write it so it will be
updated if valid data are present.

src/dtgtk/thumbnail.c

@@ -695,8 +695,12 @@ static gboolean _event_image_draw(GtkWidget *widget,
       cairo_surface_t *img_surf = NULL;
       if(thumb->zoomable)
       {
-        if(thumb->zoom > 1.0f)
-          thumb->zoom = MIN(thumb->zoom, dt_thumbnail_get_zoom100(thumb));
+        const float zoom100 = dt_thumbnail_get_zoom100(thumb);
+        // Any zoom100 > 1 is ensured to be correct
+
+        if(thumb->zoom > 1.0f && zoom100 > 1.0f)
+          thumb->zoom = MIN(thumb->zoom, zoom100);
+
         res = dt_view_image_get_surface(thumb->imgid,
                                         image_w * thumb->zoom,
                                         image_h * thumb->zoom,
@@ -2259,11 +2263,18 @@ float dt_thumbnail_get_zoom100(dt_thumbnail_t *thumb)
       (float)(thumb->height - thumb->img_margin->top - thumb->img_margin->bottom);
     const float used_w =
       (float)(thumb->width - thumb->img_margin->left - thumb->img_margin->right);
-    thumb->zoom_100 = fmaxf((float)w / used_w, (float)h / used_h);
-    if(thumb->zoom_100 < 1.0f) thumb->zoom_100 = 1.0f;
+    const float zoom100 = fmaxf((float)w / used_w, (float)h / used_h);
+
+    /* the thumb->zoom_100 value is kept for the thumbs lifetime so
+       we have to make sure this is only done if valid even in cornercases.
+       If not safe we won't keep it.
+    */
+    const gboolean safe = zoom100 >= 1.0f && thumb->img_width > 0;
+    if(safe)
+      thumb->zoom_100 = MAX(zoom100, 1.0f);
   }
 
-  return thumb->zoom_100;
+  return MAX(1.0f, thumb->zoom_100);
 }
 
 float dt_thumbnail_get_zoom_ratio(dt_thumbnail_t *thumb)