Fix compilation error and test on intel 32-bit

[Matherunner]

This PR fixes the following.

Set

export GOARCH=386

Then run go test ./..., it will fail:

./message.go:790:14: cannot convert maxSegmentSize (constant 4294967288 of type Size) to type int

After fixing the compilation error, this test will fail:

--- FAIL: TestNextAlloc (0.00s)
    message_test.go:909: max size: nextAlloc(0, 9223372036854775800, 4294967288) = _, capnp: alloc 4294967288 bytes: too large; want >=4294967288, <nil>

Anecdotally, the fixed version on a 32-bit Windows application runs fine when encoding, decoding, and doing basic RPC.

[zenhack]

LGTM. Bonus points if you want to add 386 to our CI, so it catches word size problems in the future.

[Matherunner]

LGTM. Bonus points if you want to add 386 to our CI, so it catches word size problems in the future.

Good idea. Added 386 to the matrix and tested on a PR on my fork.

[lthibault]

@zenhack CI seems to be failing on the race condition we discovered a few days ago. I'm OK to merge this, since it's definitely not causing any new issues.

[lthibault]

P.S. @Matherunner : thank you very much for this fix! 🙂

[zenhack]

Yeah, I agree; that's an existing failure and this doesn't touch anything but the tests so it's pretty low risk. Merging. Thanks @Matherunner!