Do not accept drop of location is unmounted

Author: Jeremy Wootten

libcore/FileUtils.vala

@@ -758,6 +758,10 @@ namespace Files.FileUtils {
             return Gdk.DragAction.DEFAULT;
         }
 
+        if (!dest.is_mounted) {
+            return Gdk.DragAction.DEFAULT;
+        }
+
         if (dest.is_folder ()) {
             if (!dest.is_writable ()) {
                 actions = Gdk.DragAction.DEFAULT;