Fix old bug in contrib/sslinfo: X509_NAME_to_text freed the BIO_s_mem buffer

Fix old bug in contrib/sslinfo: X509_NAME_to_text freed the BIO_s_mem buffer
it was using too soon.  In a situation where pg_do_encoding_conversion is
a no-op, this led to garbage data returned.

In HEAD, also modify the code that's ensuring null termination to make it
a tad more obvious what's happening.