window: Ignore requests to be placed on non-existent workspaces

When an X11 window requests an initial workspace, we currently trust it that the workspace actually exists. However dynamic workspaces make this easy to get wrong for applications: They make it likely for the number of workspaces to change between application starts, and if the app blindly applies its saved state on startup, it will trigger an assertion.

Make sure that we pass valid parameters to set_workspace_state(), and simply let the workspace assignment fall through to the default handling otherwise.

Fixes #1029 (closed)