ListenAndServeTLS doesn't require cert and key file names if the
server's TLSConfig has a cert configured. This code was never updated
when the GetCertificate hook was added to *tls.Config, however.

Fixes #14268

Change-Id: Ib282ebb05697edd37ed8ff105972cbd1176d900b
Reviewed-on: https://go-review.googlesource.com/19381Reviewed-by: Russ Cox <rsc@golang.org>