net: sockets: Fix TLS_HOSTNAME option length inconsistency

The TLS_HOSTNAME socket option expects a NULL terminated string and
doesn't really care about the optlen provided. However, as the option
expects that the string is NULL terminated, the optlen value should take
NULL character into account, for consistency across the codebase.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>