Ports are a maximum of uint16, but we have a few places in the recent tproxy
code where we were parsing them as 64-bit wide integers and then downcasting
them to `int`, which is technically unsafe and triggers code scanning alerts. In
practice we've validated the range elsewhere and don't build for 32-bit
platforms. This changeset fixes the parsing to make everything a bit more robust
and silence the alert.
Fixes: https://github.com/hashicorp/nomad-enterprise/security/code-scanning/444