Verify that system dependent strings in .mo files are NUL terminated.
Reported by Jakub Wilk <jwilk@jwilk.net> in
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876498>.
* gettext-runtime/intl/loadmsgcat.c (_nl_load_domain): Bail out if some of the
system-dependent strings has a last static segment that is not NUL terminated.
* gettext-tools/src/read-mo.c (get_sysdep_string): Likewise.
