coap-server.c: Fix warnings on 32-bit architectures
coap-server.c: Fix warnings on 32-bit architectures

Casting an unsigned long to void * caused a compiler warning
on 32-bit architectures. This change uses unsigned int as
this has a sufficient range for the respective calculation
and introduces a union to avoid the explicit type cast.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap