Crash in check_pos_arg

[clayton-shopify]

The following input causes a crash:

'%a%1$' % 0

I believe the bug may be here, where an argument to mrb_raisef seems to be missing:

mruby/mrbgems/mruby-sprintf/src/sprintf.c

Line 158 in 4325707

mrb_raisef(mrb, E_ARGUMENT_ERROR, "numbered(%S) after unnumbered(%S)", mrb_fixnum_value(n));

This issue was reported by https://hackerone.com/mg36