Fix null deref on writing OpenSSH pubkey with no comment.

If we're called on to generate the one-line OpenSSH public key format
for a key that we don't have a comment field for, we were mistakenly
testing this by checking if '*comment' rather than 'comment' was zero,
i.e. if comment was NULL we'd dereference it by mistake.

sshpubk.c

@@ -1502,7 +1502,7 @@ static char *ssh2_pubkey_openssh_str_internal(const char *comment,
         i += n;
         p += 4;
     }
-    if (*comment) {
+    if (comment) {
         *p++ = ' ';
         strcpy(p, comment);
     } else