Make sure the argument to cache_method is always rooted even if ano…

…ther thread

updated `m->sig`.

This causes a corruption with multi threading

src/gf.c

@@ -1038,7 +1038,10 @@ static jl_function_t *jl_mt_assoc_by_type(jl_methtable_t *mt, jl_datatype_t *tt,
  JL_GC_POP();
  return func;
  }
- jl_function_t *res = cache_method(mt, tt, func, m->sig, jl_emptysvec, m->isstaged);
+ // make sure the argument is rooted in `cache_method`
+ // in case another thread changed it.
+ newsig = m->sig;
+ jl_function_t *res = cache_method(mt, tt, func, newsig, jl_emptysvec, m->isstaged);
  JL_GC_POP();
  return res;
  }