supply proper input buffer to X509_check_host

CVE-2020-7042 use of uninitialized memory in X509_check_host is fixed with
this commit

the uninitialized buffer common_name was passed as argument to X509_check_host
which prevented proper host name validation when openssl >= 1.0.2 was in use.
This came in with #282 which went into openfortivpn 1.7.1.
Unfortunately, this problem has stayed unnoticed because the return value
was not properly checked either (which is a separate issue, with CVE-2020-7041,
and which has been fixed by the previous commit)