Fix BuddyAllocator::difference

This did not lead to a bug because we only use this in situations where
both the old and new region are the same length

src/tree_store/page_store/bitmap.rs

@@ -263,7 +263,7 @@ impl<'a, 'b> Iterator for U64GroupedBitmapDifference<'a, 'b> {
         self.data_index += 1;
         while self.data_index < self.data.len() {
             let next = self.data[self.data_index];
-            let exclusion = self.exclusion_data[self.data_index];
+            let exclusion = *self.exclusion_data.get(self.data_index).unwrap_or(&0);
             let next = next & (!exclusion);
             if next != 0 {
                 self.current = next;

src/tree_store/page_store/buddy_allocator.rs

@@ -225,13 +225,14 @@ impl BuddyAllocator {
         result
     }
 
+    // Returns all allocated pages in self, for the given region, that are not allocated in `other`
     pub(crate) fn difference(
         &self,
         region: u32,
         other: &BuddyAllocator,
         output: &mut Vec<PageNumber>,
     ) {
-        let num_pages = other.len();
+        let num_pages = self.len();
 
         for order in 0..=self.max_order {
             let other_allocated = other.get_order_allocated(order);