Improve the Iterator for IPv6 options

- Add a helper function for returning an error in the implementation of
   next
 - Add an assert in Ipv6OptionsIterator::new that ensures that an
   iterator is not created with a length field larger than that of the
   data buffer we're iterating over.

Closes: #230
Approved by: dlrobertson

src/wire/ipv6option.rs

@@ -292,7 +292,15 @@ pub struct Ipv6OptionsIterator<'a> {
 }
 
 impl<'a> Ipv6OptionsIterator<'a> {
+    /// Create a new `Ipv6OptionsIterator`, used to iterate over the
+    /// options contained in a IPv6 Extension Header (e.g. the Hop-by-Hop
+    /// header).
+    ///
+    /// # Panics
+    /// This function panics if the `length` provided is larger than the
+    /// length of the `data` buffer.
     pub fn new(data: &'a [u8], length: usize) -> Ipv6OptionsIterator<'a> {
+        assert!(length <= data.len());
         Ipv6OptionsIterator {
             pos: 0,
             hit_error: false,
@@ -301,6 +309,16 @@ impl<'a> Ipv6OptionsIterator<'a> {
     }
 }
 
+impl<'a> Ipv6OptionsIterator<'a> {
+    /// Helper function to return an error in the implementation
+    /// of `Iterator`.
+    #[inline]
+    fn return_err(&mut self, err: Error) -> Option<Result<Repr<'a>>> {
+        self.hit_error = true;
+        Some(Err(err))
+    }
+}
+
 impl<'a> Iterator for Ipv6OptionsIterator<'a> {
     type Item = Result<Repr<'a>>;
 
@@ -315,15 +333,13 @@ impl<'a> Iterator for Ipv6OptionsIterator<'a> {
                             self.pos += repr.buffer_len();
                             Some(Ok(repr))
                         }
-                        err => {
-                            self.hit_error = true;
-                            Some(err)
+                        Err(e) => {
+                            self.return_err(e)
                         }
                     }
                 }
                 Err(e) => {
-                    self.hit_error = true;
-                    Some(Err(e))
+                    self.return_err(e)
                 }
             }
         } else {
@@ -500,4 +516,11 @@ mod test {
             }
         }
     }
+
+    #[test]
+    #[should_panic(expected = "length <= data.len()")]
+    fn test_options_iter_truncated() {
+        let options = [0x01, 0x02, 0x00, 0x00];
+        let _ = Ipv6OptionsIterator::new(&options, 5);
+    }
 }