argp: Don't rely on undefined behaviour of _tolower().

Patch by Eric Blake
<https://lists.gnu.org/archive/html/bug-gnulib/2009-09/msg00287.html

>.

* argp/argp-help.c (hol_entry_cmp): Don't use _tolower on values that are
not upper-case.  Pass correct range to tolower.

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