Address MSVC 64-bit compile warnings.

[toojays]

MSVC 2015 emitted a handful of truncation errors about implicitly converting
from a size_t length argument to a uint32 or long. These all look intentional or
benign. This commit adds casts to silence the warnings.

The errors addressed are:

3>external\city.cc(170): error C2220: warning treated as error - no 'object' file generated
3>external\city.cc(170): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(183): warning C4267: 'argument': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(187): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(202): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(307): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(425): warning C4267: 'initializing': conversion from 'size_t' to 'long', possible loss of data

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[toojays]

I'm now covered by a corporate agreement.

[googlebot]

CLAs look good, thanks!

[lord-ne]

This may have been superceded by the most recently merged pull request?