Call SSLfatal when the generate_ticket_cb returns 0
Call SSLfatal when the generate_ticket_cb returns 0

Otherwise, the state machine ends up being in a bad state:
```
SSL routines:write_state_machine:missing fatal:ssl/statem/statem.c:XXX:
```

Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
Reviewed-by: Ben Kaduk <b34e9dc3dd24827b45bb22e8b76cde1bcb413b0e@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/15487)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl