argp: fix pointer-subtraction bug

* argp/argp-help.c (hol_append): Don't subtract pointers to
different arrays, as this can run afoul of -fcheck-pointer-bounds.
See the thread containing Bruno Haible's report in:
http://lists.gnu.org/archive/html/bug-gnulib/2017-05/msg00171.html



Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>