Fix crash when outgoing command cannot be found in igtlioConnector::GetOutgoingCommand (#59)
Fix crash when outgoing command cannot be found in igtlioConnector::GetOutgoingCommand (#59)

Also:
- When searching for Outgoing commands, clientID of -1 will return a match for any clientID
- Only cancel command if it is in progress
- Fix a rare crash when running PruneCompletedCommands
IGSIO/OpenIGTLinkIO,IGSIO/OpenIGTLinkIO