coap_session.c: Fix GCC array bounds warning for memcpy

Copying from the multidimensional coap_layers_coap array into the
coap_layers of endpoint::sock.lfunc triggers a warning from
GCC v13.2.0 as the global variable has no space for the end marker
COAP_PROTO_LAST.

This change adds an assertion to indicate that proto is always less
than COAP_PROTO_LAST.

src/coap_session.c

@@ -1756,6 +1756,7 @@ coap_new_endpoint(coap_context_t *context, const coap_address_t *listen_addr, co
   ep->context = context;
   ep->proto = proto;
   ep->sock.endpoint = ep;
+  assert(proto < COAP_PROTO_LAST);
   memcpy(&ep->sock.lfunc, coap_layers_coap[proto], sizeof(ep->sock.lfunc));
 
   if (COAP_PROTO_NOT_RELIABLE(proto)) {