fix use-after-scope problems in BoundedAsyncPipeTest
fix use-after-scope problems in BoundedAsyncPipeTest

Summary:
BoundedAsyncPipe::write() accepts arguments by-reference, so even
these constants may be out-of-scope by the time the coroutine actually runs,
causing use-after-scope problems which are detected by ASAN.

Wrapping in co_invoke allows us to scope the parameter so that it
lives long enough.

Reviewed By: lxfind

Differential Revision: D26649913

fbshipit-source-id: 5bc6b7f64a5e75c9386b245fa3fd1484efaf1d13

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