[gimp] app: fix incompatible pointer type warning in gimpviewrendererdrawable.c

[Ell <ell src gnome org>]

commit a5c8dd5c7373b96d0234fe03a23fee068978ab1e
Author: Ell <ell_se yahoo com>
Date:   Tue Sep 18 16:17:36 2018 -0400

    app: fix incompatible pointer type warning in gimpviewrendererdrawable.c
    
    ... more GLib 2.58 fallout.

 app/widgets/gimpviewrendererdrawable.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/app/widgets/gimpviewrendererdrawable.c b/app/widgets/gimpviewrendererdrawable.c
index d2e7013ae0..4b35ed1650 100644
--- a/app/widgets/gimpviewrendererdrawable.c
+++ b/app/widgets/gimpviewrendererdrawable.c
@@ -379,7 +379,13 @@ gimp_view_renderer_drawable_cancel_render (GimpViewRendererDrawable *renderdrawa
    * after the drawable had died, and our completion callback is prepared to
    * handle cancelation.
    */
-  g_clear_pointer (&renderdrawable->priv->render_async, gimp_cancelable_cancel);
+  if (renderdrawable->priv->render_async)
+    {
+      gimp_cancelable_cancel (
+        GIMP_CANCELABLE (renderdrawable->priv->render_async));
+
+      renderdrawable->priv->render_async = NULL;
+    }
 
   g_clear_object (&renderdrawable->priv->render_widget);
 }