parthelper: fix compilation without optimization

parthelper: fix compilation without optimization

Daniel's patch works with gcc and CFLAGS containing -O (the
autoconf default), but fails with non-gcc or with other
CFLAGS (such as -g), since c-ctype.h declares c_isdigit as
a macro only for certain compilation settings.

* src/Makefile.am (libvirt_parthelper_LDFLAGS): Add gnulib
library, for when c_isdigit is not a macro.
* src/storage/parthelper.c (main): Avoid out-of-bounds
dereference, noticed by Jim Meyering.