Some platforms define NULL as ((void *)0). Unfortunately, a void*

can't be used as a function pointer according the the standards.  Use
a 0 instead and there will be no trouble.

crypto/stack/stack.c

@@ -111,7 +111,7 @@ err:
 
 STACK *sk_new_null(void)
 	{
-	return sk_new((int (*)(const char * const *, const char * const *))NULL);
+	return sk_new((int (*)(const char * const *, const char * const *))0);
 	}
 
 STACK *sk_new(int (*c)(const char * const *, const char * const *))