Fix to not print unnecessary comma.

When `n2->car` is 0 and `n2->cdd` is not 0,
unnecessary comma was printed before first local variables.
revert #2344 because of #2395
Merge pull request #2333 from yui-knk/fix/dump

Fix to not print unnecessary comma.
Merge branch 'master' of github.com:mruby/mruby