Fix checkusm check for UDP

From the RFC:
> If the computed  checksum  is zero,  it is transmitted  as all ones (the
> equivalent  in one's complement  arithmetic).   An all zero  transmitted
> checksum  value means that the transmitter  generated  no checksum  (for
> debugging or for higher level protocols that don't care).

Fixes #460 .

src/wire/udp.rs

@@ -115,6 +115,14 @@ impl<T: AsRef<[u8]>> Packet<T> {
             return true;
         }
 
+        // From the RFC:
+        // > An all zero transmitted checksum value means that the transmitter
+        // > generated no checksum (for debugging or for higher level protocols
+        // > that don't care).
+        if self.checksum() == 0 {
+            return true;
+        }
+
         let data = self.buffer.as_ref();
         checksum::combine(&[
             checksum::pseudo_header(src_addr, dst_addr, IpProtocol::Udp, self.len() as u32),
@@ -373,6 +381,18 @@ mod test {
         assert_eq!(packet.checksum(), 0xffff);
     }
 
+    #[test]
+    #[cfg(feature = "proto-ipv4")]
+    fn test_no_checksum() {
+        let mut bytes = vec![0; 8];
+        let mut packet = Packet::new_unchecked(&mut bytes);
+        packet.set_src_port(1);
+        packet.set_dst_port(31881);
+        packet.set_len(8);
+        packet.set_checksum(0);
+        assert!(packet.verify_checksum(&SRC_ADDR.into(), &DST_ADDR.into()));
+    }
+
     #[cfg(feature = "proto-ipv4")]
     fn packet_repr() -> Repr {
         Repr {