Safe stack reorganisation in terms of function casts.

After some messing around this seems to work but needs
a few more tests. Working out the syntax for sk_set_cmp_func()
(cast it to a function that itself returns a function pointer)
was painful :-(

Needs some testing to see what other compilers think of this
syntax.

Also needs similar stuff for ASN1_SET_OF etc etc.

crypto/stack/stack.c

@@ -109,6 +109,11 @@ err:
 	return(NULL);
 	}
 
+STACK *sk_new_null(void)
+	{
+	return sk_new((int (*)(const char * const *, const char * const *))NULL);
+	}
+
 STACK *sk_new(int (*c)(const char * const *, const char * const *))
 	{
 	STACK *ret;