fix invalid memory access problem

Continuation of https://github.com/matsumoto-r/ngx_mruby/issues/18

nginx use unaddressable bytes when mruby's value is passed to nginx.

Though I don't know in detail, nginx somtimes uses unaddressable bytes when mruby's value(for example, return value of mrb_strdup) is passed to nginx.(Is cause GC?)

I think it is good that using the memory area of nginx instead of the memory area of mruby  when possible.