argp-standalone: restrict value range passed to isprint function

According to C standards isprint argument shall be representable as an
unsigned char or be equal to EOF, otherwise the behaviour is undefined.

Passing arbitrary ints leads to segfault in nm program from elfutils.

Restrict isprint argument range to values representable by unsigned char.

Note: a similar change was done to the internal argp implementation of
glibc in commit
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=9055724a92433ffa4c36f93d918ee1b3dfa1d6f7

.

[Thomas: add a reference to the corresponding glibc fix.]

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>