[simple-scan] Do not override existing file if user does not want to do it.

[Robert Ancell <rancell src gnome org>]

commit 64a9320e05b0ff562ecdcd46dcf58b94128b894a
Author: Victor Mireyev <victor opennodecloud com>
Date:   Mon Feb 18 16:51:09 2019 +0300

    Do not override existing file if user does not want to do it.

 src/app-window.vala | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/app-window.vala b/src/app-window.vala
index 6e46970..af20c40 100644
--- a/src/app-window.vala
+++ b/src/app-window.vala
@@ -443,6 +443,8 @@ public class AppWindow : Gtk.ApplicationWindow
 
             if (check_overwrite (save_dialog.transient_for, files))
                 break;
+            else
+                uri = null;
         }
 
         settings.set_string ("save-directory", save_dialog.get_current_folder ());