Fix potential out-of-bounds memory access in cpp starter

[djkeyes]

Hey, I noticed I introduced a bug in PR #113 . Specifically, the pointer returned by return &citytiles.back(); can become invalid if you add a bunch more citytiles to a city, and vector has to resize the underlying memory. The fix is to lookup the memory addresses after the vector is done being modified.

Let me know if there's anything you'd like me to change.

• To test this PR, I manually added some assertions. Is there a place for automated API tests, or are we sticking to manual testing for now?
• Do you want me to re-zip the starter packages?

[StoneT2000]

I can handle the rezipping. We usually run npm test to test, if someone wants to, we'd be very happy to have a circleci integration :)