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

src/class.c

@@ -612,6 +612,9 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
                 *p = (mrb_int)f;
               }
               break;
+            case MRB_TT_STRING:
+              mrb_raise(mrb, E_TYPE_ERROR, "String can't be coerced into int");
+              break;
             default:
               *p = mrb_fixnum(mrb_Integer(mrb, *sp));
               break;