vm_backtrace.c: use rb_id2str

* vm_backtrace.c (location_base_label): convert ID to string
  directly, and the return value is frozen.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50307 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_backtrace.c

@@ -208,7 +208,7 @@ location_base_label(rb_backtrace_location_t *loc)
       case LOCATION_TYPE_ISEQ_CALCED:
 	return loc->body.iseq.iseq->location.base_label;
       case LOCATION_TYPE_CFUNC:
-	return rb_sym_to_s(ID2SYM(loc->body.cfunc.mid));
+	return rb_id2str(loc->body.cfunc.mid);
       case LOCATION_TYPE_IFUNC:
       default:
 	rb_bug("location_base_label: unreachable");