Fix intermittent s_server issues with ECDHE

Resolve a problem when using s_server with ECDHE cipher
suites in OpenSSL_1_0_1-stable.  Due to an uninitialized variable,
SSL_CTX_set_tmp_ecdh() is not always invoked within s_server. This bug
appears to have been introduced by
05990777

.

Reviewed-by: Tim Hudson <tjh@openssl.org>