placessidebar: do not crash if uris is NULL

On Drag'n'Drop actions across system boundaries (VM host to guest), the
happen to be null.

https://bugzilla.gnome.org/show_bug.cgi?id=757298

gtk/gtkplacessidebar.c

@@ -1715,7 +1715,7 @@ build_file_list_from_uris (const gchar **uris)
   gint i;
 
   result = NULL;
-  for (i = 0; uris[i]; i++)
+  for (i = 0; uris && uris[i]; i++)
     {
       GFile *file;