Don't add 'gnulib-lib' to include path when compiling libgettextpo tests.
po-gram: fix memory leaks
libintl: Fix pointer use after free and make error handling robuster

Previously, _nl_find_msg had a use of pointer after free'd.  Also,
some callers of the function didn't check the failure.

Merge changes from glibc.  Reported by Carlos O'Donell in
<https://savannah.gnu.org/bugs/?38930>.