gpg: Avoid endless loop if a card's serial number can't be read.

* g10/skclist.c (enum_secret_keys): Move list forward on error.
--

The error is not easy to reproduce but may occur if a card is removed
at the wrong time.  Tested by changing the code.

Signed-off-by: Werner Koch <wk@gnupg.org>

g10/skclist.c

@@ -423,6 +423,7 @@ enum_secret_keys (ctrl_t ctrl, void **context, PKT_public_key *sk)
                           if (opt.verbose)
                             log_info (_("error getting serial number of card: %s\n"),
                                       gpg_strerror (err));
+                          c->sl = c->sl->next;
                           continue;
                         }