Fix memory leak in mosquitto_tls_opts_set()

When calling mosquitto_tls_opts_set() multiple time in a row
on the same mosquitto context (Example: trying to reconnect
to mqtt broker), some of the tls options are not free, resulting in a memory leak.

The TLS options (tls_version, tls_ciphers) must be free before being set.

Signed-off-by: Guillaume PELLEGRINO <guillaume.pellegrino@yahoo.com>