Fix error handling in be_tls_open_server()

Hi,

A static analyzer reported a possible pfree(NULL) in 
be_tls_open_server(). Here is a fix. Also handle an error from 
X509_NAME_print_ex().

AFAICS, the error "SSL certificate's distinguished name contains 
embedded null" could not be reached at all, because XN_FLAG_RFC2253 
passed to X509_NAME_print_ex() ensures that null bytes are escaped.

Best regards,

-- 
Sergey Shinderuk        https://postgrespro.com/