Added verification of PipeServer connection state

[glaysons]

When testing in a multi-threaded environment with many connections per second, the following error was being generated: Cannot access a closed pipe.

A brief adjustment was made to check the connection status before attempting to close the connection.

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[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.