core::iter: fix bug uncovered by arith-overflow.

(The bug was in `impl RandomAccessIterator for Rev`; it may or may not
have been innocuous, depending on what guarantees one has about the
behavior of `idx` for the underlying iterator.)

src/libcore/iter.rs

@@ -1130,7 +1130,11 @@ impl<I> RandomAccessIterator for Rev<I> where I: DoubleEndedIterator + RandomAcc
     #[inline]
     fn idx(&mut self, index: usize) -> Option<<I as Iterator>::Item> {
         let amt = self.indexable();
-        self.iter.idx(amt - index - 1)
+        if amt > index {
+            self.iter.idx(amt - index - 1)
+        } else {
+            None
+        }
     }
 }