fix compare_pstr_to_string (#2148)

src/machine/partial_string.rs

@@ -68,7 +68,7 @@ pub struct HeapPStrIter<'a> {
     stepper: fn(&mut HeapPStrIter<'a>) -> Option<PStrIteratee>,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub struct PStrPrefixCmpResult {
     pub focus: usize,
     pub offset: usize,
@@ -193,7 +193,7 @@ impl<'a> HeapPStrIter<'a> {
             }
         }
 
-        final_result
+        Some(result)
     }
 
     fn walk_hare_to_cycle_end(&mut self) {