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/useradd.c

@@ -894,7 +894,7 @@ static int get_groups (char *list)
 	close_group_files ();
 	unlock_group_files ();
 
-	user_groups[ngroups] = (char *) 0;
+	user_groups[ngroups] = NULL;
 
 	/*
 	 * Any errors in finding group names are fatal
@@ -2546,7 +2546,7 @@ int main (int argc, char **argv)
 	/*
 	 * Initialize the list to be empty
 	 */
-	user_groups[0] = (char *) 0;
+	user_groups[0] = NULL;
 
 
 	is_shadow_pwd = spw_file_present ();