Clear all old flags, even if we change the gc mode during the major gc.
fixed wrong loop termination introduced by 79d38c4
Merge pull request #745 from authorNari/clear-old-flag-to-change-gc-mode

Clear all old flags, even if we change the gc mode during the major gc.
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' into iij