regex: port to hosts where malloc (0) == NULL Reported by Aharon Robbins in <http://sourceware.org/ml/libc-alpha/2012-12/msg00456.html>. * lib/regex_internal.c (re_node_set_alloc): Don't assume that malloc (0) yields nonnull. * lib/regex_internal.h (MALLOC_0_IS_NONNULL): New macro. * m4/regex.m4 (gl_PREREQ_REGEX): Require gl_EEMALLOC. * modules/regex (Files): Add m4/eealloc.m4.