pdu.c: fix len returned by coap_get_data() for empty payload

John Carter has provided this (slightly edited) patch to set len to 0
for PDUs that do not contain a payload. This makes this function more
robust for callers that do not care about the function result.

This change fixes issue 120 [1].

[1] https://github.com/obgm/libcoap/issues/120

src/pdu.c

@@ -324,9 +324,15 @@ coap_get_data(coap_pdu_t *pdu, size_t *len, uint8_t **data) {
   assert(len);
   assert(data);
 
-  *len = pdu->used_size - (pdu->data - pdu->token);
   *data = pdu->data;
-  return *data != NULL;
+  if(pdu->data == NULL) {
+     *len = 0;
+     return 0;
+  }
+
+  *len = pdu->used_size - (pdu->data - pdu->token);
+
+  return 1;
 }
 
 #ifndef SHORT_ERROR_RESPONSE