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>

libmisc/sulog.c

@@ -64,7 +64,7 @@ void sulog (const char *tty, bool success, const char *oldname, const char *name
 		/* Do not return if the group permission were raised. */
 		exit (EXIT_FAILURE);
 	}
-	if (fp == (FILE *) 0) {
+	if (fp == NULL) {
 		return;		/* can't open or create logfile */
 	}