Let Gnulib deal with malloc/realloc 0

Let Gnulib deal with malloc/realloc 0

Instead of worrying about whether malloc and realloc yield a
non-null pointer when given size 0 (the traditional behavior
recommended by Doug McIlroy, and the only behavior that makes
sense for Emacs), let Gnulib arrange for the traditional behavior.
* admin/merge-gnulib (GNULIB_MODULES): Add malloc-gnu (which we
were already depending on indirectly) and realloc-posix.
* lib/gnulib.mk.in, m4/gnulib-comp.m4:
Regenerate by running admin/merge-gnulib.

(cherry picked from commit 29794c71452c58c596c58fd6148741d213d3ee7b)