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/newusers.c

@@ -1088,7 +1088,7 @@ int main (int argc, char **argv)
 	 * over 100 is allocated. The pw_gid field will be updated with that
 	 * value.
 	 */
-	while (fgets (buf, (int) sizeof buf, stdin) != (char *) 0) {
+	while (fgets (buf, (int) sizeof buf, stdin) != NULL) {
 		line++;
 		cp = strrchr (buf, '\n');
 		if (NULL != cp) {