Fix packet buffer panic caused by large payload (#332)

When packet buffer's payload buffer does not have enough contiguous
window left, the ring buffer roll over uses an incorrect size
causing the ring buffer pointer not resetting to the head.

When the payload enqueued is larger than 1/2 of the payload ring
buffer, this bug will cause the slice returned by
`PacketBuffer::enqueue` to not match the requested size, and
trigger `debug_assert` in debug profile or size mismatch panic in
`copy_from_slice` when compiled in release profile.

src/storage/packet_buffer.rs

@@ -96,8 +96,8 @@ impl<'a, 'b, H> PacketBuffer<'a, 'b, H> {
             } else {
                 // Add padding to the end of the ring buffer so that the
                 // contiguous window is at the beginning of the ring buffer.
-                *self.metadata_ring.enqueue_one()? = PacketMetadata::padding(size);
-                self.payload_ring.enqueue_many(size);
+                *self.metadata_ring.enqueue_one()? = PacketMetadata::padding(contig_window);
+                self.payload_ring.enqueue_many(contig_window);
             }
         }
 
@@ -224,6 +224,14 @@ mod test {
         assert_eq!(buffer.metadata_ring.len(), 0);
     }
 
+    #[test]
+    fn test_padding_with_large_payload() {
+        let mut buffer = buffer();
+        assert!(buffer.enqueue(12, ()).is_ok());
+        assert!(buffer.dequeue().is_ok());
+        buffer.enqueue(12, ()).unwrap().copy_from_slice(b"abcdefghijkl");
+    }
+
     #[test]
     fn test_dequeue_with() {
         let mut buffer = buffer();