don't overwrite backtrace info.

src/backtrace.c

@@ -215,11 +215,14 @@ packed_backtrace(mrb_state *mrb)
 void
 mrb_keep_backtrace(mrb_state *mrb, mrb_value exc)
 {
+  mrb_sym sym = mrb_intern_lit(mrb, "backtrace");
   mrb_value backtrace;
-  int ai = mrb_gc_arena_save(mrb);
+  int ai;
 
+  if (mrb_iv_defined(mrb, exc, sym)) return;
+  ai = mrb_gc_arena_save(mrb);
   backtrace = packed_backtrace(mrb);
-  mrb_iv_set(mrb, exc, mrb_intern_lit(mrb, "backtrace"), backtrace);
+  mrb_iv_set(mrb, exc, sym, backtrace);
   mrb_gc_arena_restore(mrb, ai);
 }
 

src/error.c

@@ -200,6 +200,7 @@ exc_debug_info(mrb_state *mrb, struct RObject *exc)
   mrb_callinfo *ci = mrb->c->ci;
   mrb_code *pc = ci->pc;
 
+  if (mrb_obj_iv_defined(mrb, exc, mrb_intern_lit(mrb, "file"))) return;
   while (ci >= mrb->c->cibase) {
     mrb_code *err = ci->err;