Fixed random crash when creating client

[mattbeghin]

In MadMapper, when creating / deleting the syphon client multiple times per second for 5 minutes while server is pushing frames at 60 FPS, I generally get a crash because we get a new frame from server before "SyphonClientBase::_handler" variable is initialized. This patch solved the issue.

[bangnoise]

Hey - thanks for this. I don't see how the handler can be being invoked before the call further down to -addInfoClient:isFrameClient:. I will try to recreate the issue.