tc-testing: tdc.py: Guard against lack of returncode in executed command

Add some defensive coding in case one of the subprocesses created by tdc
returns nothing. If no object is returned from exec_cmd, then tdc will
halt with an unhandled exception.

Signed-off-by: Brenda J. Butler <bjb@mojatatu.com>
Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>