[perl #123245] tests for format crashes
[perl #123245] avoid a panic in sv_chop() in formats

This fixes two issues:

1) if you don't supply enough arguments to the format, pp_formline()
  uses &PL_sv_no as the sv, since we've already warned about the
  missing format argument, we don't need to produce a read only error
  for an SV the caller didn't supply

2) when the supplied string is empty for FF_LINESNGL and FF_LINEGLOB
  the case would skip most of its processing, including setting
  chophere, this meant that when the following FF_CHOP operator was
  processed it would pass a pointer into a different string, producing
  a panic.