Add comment to sshkey.c to explain commit 66d2e6

Add a corresponding comment in sshkey.c to clarify the reason for a
previous commit, so that future review doesn't need to reference the
commit message.

sshkey.c

@@ -2761,6 +2761,11 @@ sshkey_private_to_blob2(struct sshkey *prv, struct sshbuf *blob,
  kdfname = "none";
  } else if (ciphername == NULL)
  ciphername = DEFAULT_CIPHERNAME;
+ /*
+ * NOTE: Without OpenSSL, this string comparison is still safe, even
+ * though it will never match because the multithreaded cipher is not
+ * enabled.
+ */
  else if (strcmp(ciphername, "chacha20-poly1305-mt@hpnssh.org") == 0)
  ciphername = "chacha20-poly1305@openssh.com";
  if ((cipher = cipher_by_name(ciphername)) == NULL) {