regex: port to hosts where malloc (0) == NULL

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.