Call NULL by its name

In variadic functions we still do the cast.  In POSIX, it's not
necessary, since NULL is required to be of type 'void *', and 'void *'
is guaranteed to have the same alignment and representation as 'char *'.
However, since ISO C still doesn't mandate that, and moreover they're
doing dubious stuff by adding nullptr, let's be on the cautious side.
Also, C++ requires that NULL is _not_ 'void *', but either plain 0 or
some magic stuff.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/groupmod.c

@@ -248,7 +248,7 @@ static void grp_update (void)
 			if (NULL != grp.gr_mem[0])
 				gr_free_members(&grp);
 			grp.gr_mem = (char **)xmalloc(sizeof(char *));
-			grp.gr_mem[0] = (char *)0;
+			grp.gr_mem[0] = NULL;
 		} else {
 			// append to existing groups
 			if (NULL != grp.gr_mem[0])