rustls: cap maximum allowed CRL file size to 8MB

Allowing 4GB on a 32-bit system is just asking for problems and could in
theory cause integer overflow in the dynbuf code.

The dynbuf now has an assert to catch code trying to set a max larger
than half SIZE_T_MAX.

Reported-by: Rinku Das
Closes #16716

lib/dynbuf.c

@@ -43,6 +43,7 @@ void Curl_dyn_init(struct dynbuf *s, size_t toobig)
 {
   DEBUGASSERT(s);
   DEBUGASSERT(toobig);
+  DEBUGASSERT(toobig <= MAX_DYNBUF_SIZE); /* catch crazy mistakes */
   s->bufr = NULL;
   s->leng = 0;
   s->allc = 0;