Fix SIGFPE when empty window geometry set

A client setting an empty window geometry at launch caused a SIGFPE as we wouldn't finish the resize, but still focus, meaning sending any previously sent configuration, even if there wasn't one. That meant "re"sending size 0x0, position (0, 0) and geometry scale 0, the latter causing SIGFPE.

For master I think we should just error out these clients, but maybe for stable branches it should left to g_return_if_fail() as is done here too if the old path happens to trigger again.

Also moved a incorrectly placed test case, and made it actually run, and added a test client for the above crash.