Silence warning about is_empty method

These were flagged by `cargo clippy`:

    warning: item has a public `len` method but no corresponding
             `is_empty` method

src/wire/tcp.rs

@@ -712,6 +712,7 @@ pub enum Control {
     Rst
 }
 
+#[allow(clippy::len_without_is_empty)]
 impl Control {
     /// Return the length of a control flag, in terms of sequence space.
     pub fn len(self) -> usize {

src/wire/udp.rs

@@ -27,6 +27,7 @@ mod field {
     }
 }
 
+#[allow(clippy::len_without_is_empty)]
 impl<T: AsRef<[u8]>> Packet<T> {
     /// Imbue a raw octet buffer with UDP packet structure.
     pub fn new_unchecked(buffer: T) -> Packet<T> {