Add NULL to end of g_filename_complete_get_completions() return value

g_filename_complete_get_completions() return value is meant to be a
g_strfreev-compatible array i.e. NULL-terminated. However, pointer arrays
aren't automagically NULL-terminated. This fixes bug 586868

gio/gfilenamecompleter.c

@@ -494,6 +494,8 @@ g_filename_completer_get_completions (GFilenameCompleter *completer,
 
   g_free (prefix);
 
+  g_ptr_array_add (res, NULL);
+
   return (char**)g_ptr_array_free (res, FALSE);
 }