Fix negative port detection for IPv6 addresses on 32-bit

Fix negative port detection for IPv6 addresses on 32-bit

On a 32-bit Arch, our `test_ComboAddress` unit test fails because
`ComboAddress("[::1]:-6")` is considered valid. This is caused by
`stoul()` not throwing for a negative value and returning an `unsigned
long` value using unsigned integer wraparound rules. Since we used to
store the result value in a `signed int` and treat negative values
as if the port was not set, the test failed.