mjit.c (free_list): clear .length

For robustness against future changes. There should be no impact
at the moment,here, but we may call mjit_finish more than once
in a process lifetime in the future (implementing "stop"
instead of just "pause")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65443 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -135,6 +135,7 @@ free_list(struct rb_mjit_unit_list *list)
         free_unit(node->unit);
         xfree(node);
     }
+    list->length = 0;
 }
 
 /* MJIT info related to an existing continutaion.  */