Prevent accepting String value for mrb_get_args("i")

Accepting String value for mrb_get_args("i")
causes unintentional expression.

  * `"str" * "3"` => `"strstrstr"`

This change prevents String->Integer conversion for mrb_get_args("i")
and fixes above problem.

See also:
https://github.com/mruby/mruby/issues/1903