daemon/session2: avoid incorrectly generated errors

The _try_ functions additionally return positive values on success,
and the code around didn't deal with that.

So far there's no evidence that this caused any issues beyond debug-logs:
  [prlayr] ... iteration of group ... ended with status 'Unknown error ...

daemon/session2.c

@@ -1449,6 +1449,8 @@ static int session2_transport_pushv(struct session2 *s,
 			} else {
 				int ret = uv_udp_try_send((uv_udp_t*)handle,
 						(uv_buf_t *)iov, iovcnt, comm->comm_addr);
+				if (ret > 0) // equals buffer size, only confuses us
+					ret = 0;
 				if (ret == UV_EAGAIN) {
 					ret = kr_error(ENOBUFS);
 					session2_event(s, PROTOLAYER_EVENT_OS_BUFFER_FULL, NULL);
@@ -1480,6 +1482,8 @@ static int session2_transport_pushv(struct session2 *s,
 				ret = kr_error(ENOBUFS);
 				session2_event(s, PROTOLAYER_EVENT_OS_BUFFER_FULL, NULL);
 			}
+			else if (ret > 0) // iovec_sum was checked, let's not get confused anymore
+				ret = 0;
 
 			if (false && ret == UV_EAGAIN) {
 				uv_write_t *req = malloc(sizeof(*req));