fix crash in read_term (#1616)

src/machine/machine_state.rs

@@ -536,7 +536,7 @@ impl MachineState {
 
         loop {
             match self.read(stream, &indices.op_dir) {
-                Ok(term_write_result) => {
+                Ok(mut term_write_result) => {
                     let heap_loc = read_heap_cell!(self.heap[term_write_result.heap_loc],
                         (HeapCellValueTag::PStr | HeapCellValueTag::PStrOffset) => {
                             pstr_loc_as_cell!(term_write_result.heap_loc)
@@ -568,6 +568,10 @@ impl MachineState {
                         }
                     }
 
+                    for var in term_write_result.var_dict.values_mut() {
+                        *var = heap_bound_deref(&self.heap, *var);
+                    }
+
                     let singleton_var_list = push_var_eq_functors(
                         &mut self.heap,
                         term_write_result.var_dict.iter().filter(|(_, binding)| {