Fix undefined behavior in Hole::move_to()

Author: jgallagher

src/binary_heap.rs

@@ -428,8 +428,9 @@ impl<'a, T> Hole<'a, T> {
     unsafe fn move_to(&mut self, index: usize) {
         debug_assert!(index != self.pos);
         debug_assert!(index < self.data.len());
-        let index_ptr: *const _ = self.data.get_unchecked(index);
-        let hole_ptr = self.data.get_unchecked_mut(self.pos);
+        let ptr = self.data.as_mut_ptr();
+        let index_ptr: *const _ = ptr.add(index);
+        let hole_ptr = ptr.add(self.pos);
         ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
         self.pos = index;
     }