Fix bugprone-narrowing-conversions reported by clang-tidy

Clang-tidy started reporting a couple of `bugprone-narrowing-conversions`. The check was disabled to allow an Envoy update to proceed. The code issues should be fixed and the check re-enabled.

Check disabled in [.clang-tidy](https://github.com/envoyproxy/nighthawk/blob/main/.clang-tidy).

The reported issues are in [this CI log](https://github.com/envoyproxy/nighthawk/files/7283496/reported_issues.txt).