rt: Fix box annihilator to deallocate exchange heap boxes properly.

Author: Elliott Slaughter

src/rt/rust_box_annihilator.cpp

@@ -197,7 +197,13 @@ annihilator::do_annihilate(rust_task *task, rust_opaque_box *box) {
     annihilator annihilator(task, true, tydesc->shape,
                             tydesc->shape_tables, p);
     annihilator.walk();
-    task->boxed.free(box);
+    // NB: A reference count of -1 indicates that this box lives on the
+    // exchange heap. Otherwise it lives on the task-local heap.
+    if (box->ref_count + 1 == 0) {
+        task->kernel->free(box);
+    } else {
+        task->boxed.free(box);
+    }
 }
 
 void