net.c: explicitly clear pointer to free'd storage
net.c: explicitly clear pointer to free'd storage

Static code analysis tools have incorrectly reported a memory leak
in function handle_request(). To avoid this false positive, the
variable response is explicitly set to NULL after releasing the
memory with coap_delete_pdu().

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap