ssl3_free(): Return if it wasn't created

If somewhere in SSL_new() there is a memory allocation failure, ssl3_free() can
get called with s->s3 still being NULL.

Patch also provided by Willy Tarreau <wtarreau@haproxy.com>

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Viktor Dukhovni <openssl-users@dukhovni.org>