Make base64_decode in rend_parse_client_keys more foolproof

In general, whenever we can, we should be doing
  base64_decode(buf, sizeof(buf), s, strlen(s)),
and not
  base_64_decode(buf, expr1, s, expr2)
where we hope that expr1 is a good name for the size of buf and expr2
is a good formula for the length of the base64 expression in s.

src/or/routerparse.c

@@ -5171,9 +5171,9 @@ rend_parse_client_keys(strmap_t *parsed_clients, const char *ckstr)
     /* The size of descriptor_cookie_tmp needs to be REND_DESC_COOKIE_LEN+2,
      * because a base64 encoding of length 24 does not fit into 16 bytes in all
      * cases. */
-    if ((base64_decode(descriptor_cookie_tmp, REND_DESC_COOKIE_LEN+2,
-                       tok->args[0], REND_DESC_COOKIE_LEN_BASE64+2+1)
-           != REND_DESC_COOKIE_LEN)) {
+    if (base64_decode(descriptor_cookie_tmp, sizeof(descriptor_cookie_tmp),
+                      tok->args[0], strlen(tok->args[0]))
+        != REND_DESC_COOKIE_LEN) {
       log_warn(LD_REND, "Descriptor cookie contains illegal characters: "
                "%s", escaped(tok->args[0]));
       goto err;