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.
add a few const qualifier
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
reduce mrb_funcall in exception allocation
initialize nomem_err->mesg->c before exposing to Ruby world; close #2428