ssltest.c: cb_ticket2 appears to not return a value when it "should"

cb_ticket2() does an exit, and should therefore not need to return anything.
Some compilers don't detect that, or don't care, and warn about a non-void
function without a return statement.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4713)