Fix Clang failing TestKey() test from bad pointer arithmetic.

[mikepb]

Clang fails this test on make check:

trie-test.cc:71: TestKey(): 115: Assertion `r_key.ptr() == NULL' failed.

This patch uses intptr_t for pointer arithmetic to enable clang support.

[s-yata]

Thank you. I've merged this pull request.

The behavior of a pointer arithmetic that causes an overflow is undefined.
Hmm, uintptr_t is better than intptr_t?

[mikepb]

I was thinking that intptr_t would be more "proper" because the code is using -1 to mean empty. That would sidestep underflow issues entirely.

Though now that you bring it up, using undefined behavior for pointer arithmetic seems ripe for hard-to-find bugs. I was surprised that the initializer had the intended behavior too.

[s-yata]

Thank you for your comment.
There is not a reason enough to use -1 to mean empty.