Fix LibreSSL X509 (SSL) certificate hostname checking

(Currently) LibreSSL doesn't calculate the string length of the hostname
that's passed to X509_check_host automatically in case namelen/chklen is 0.
This causes server certificate validation to fail when building MariaDB with
LibreSSL.

The proposed fix makes MariaDB determine the string length passed to
X509_check_host. As there are no ill side-effects (OpenSSL's X509_check_host
also simply calls strlen if namelen == 0, see also X509_check_host(3)), this
wasn't wrapped in any #ifdef like constructs.

Please see here for a proposed patch to modify LibreSSL's behavior:
libressl-portable/openbsd#87