BUG: Fix invalid return value check in vtkClientSocket.

In ConnectToServer(), the return value from CreateSocket() can be 0.
It returns -1 on error, not 0. The invalid check was causing
ParaView tests to fail on ICC dashboards.

Change-Id: I0e770c8889823ecd163d0c8be5ed6a9999a192bd
Merge topic 'fix_invalid_socket_check' into master

73739ae0 BUG: Fix invalid return value check in vtkClientSocket.