coap_debug.c: Fix len parameter for inet_ntop()

On RIOT, inet_ntop() expects the size parameter to reflect the
length of the given address. A larger value causes inet_ntop()
to return ENOSPC.

This change passes the address size as a workaround, or the actual
number of remaining bytes in the output buffer in case less than
address size bytes remain.

src/coap_debug.c

@@ -196,7 +196,8 @@ coap_print_addr(const struct coap_address_t *addr, unsigned char *buf, size_t le
   }
 
   /* Cast needed for Windows, since it doesn't have the correct API signature. */
-  if (inet_ntop(addr->addr.sa.sa_family, addrptr, (char *)p, len) == 0) {
+  if (inet_ntop(addr->addr.sa.sa_family, addrptr, (char *)p,
+                min(len, addr->size)) == 0) {
     perror("coap_print_addr");
     return 0;
   }