Fix illegal instruction caused by overflow in channel cloning

[KiChjang]

Fixes #35847.

r? @alexcrichton

[brson]

@bors r+

[bors]

📌 Commit 899c289 has been approved by brson

[bors]

⌛ Testing commit 899c289 with merge 2a14134...

[bors]

💔 Test failed - auto-win-gnu-32-opt-rustbuild

[alexcrichton]

@bors: retry

On Fri, Sep 2, 2016 at 8:24 AM, bors notifications@github.com wrote:

💔 Test failed - auto-win-gnu-32-opt-rustbuild
https://buildbot.rust-lang.org/builders/auto-win-gnu-32-opt-rustbuild/builds/2379

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[bors]

⌛ Testing commit 899c289 with merge 49e9bfd...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: brson
Pushing 49e9bfd to master...