Fix UnboundLocalError (subprocess died before reader threads started).

[jsiirola]

Summary/Motivation:

This resolves an exception that is due to a timing error where a subprocess can die before the I/O reader thread is launched. The cleanup routines need to verify that the threads were created before joining them.

The original error was reported on the Pyomo Forum

Changes proposed in this PR:

• verify that the reader thread was created before closing it

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[coveralls]

Coverage increased (+0.0009%) to 63.973% when pulling 85b13d0 on subprocess_th_fix into 89086c2 on master.

[coveralls]

Coverage increased (+0.0009%) to 63.973% when pulling 85b13d0 on subprocess_th_fix into 89086c2 on master.

[coveralls]

Coverage increased (+0.0009%) to 63.973% when pulling 85b13d0 on subprocess_th_fix into 89086c2 on master.

[coveralls]

Coverage increased (+0.01%) to 63.985% when pulling 85b13d0 on subprocess_th_fix into 89086c2 on master.