fix crash: urgent floating con on separate workspace (thanks Piotr)

fix crash: urgent floating con on separate workspace (thanks Piotr)

If there is a single floating con on a separate workspace that is not
focused, and this con becomes urgent, switching back to that workspace
may result in a crash of i3. This is because while setting the urgency
of parent containers, 'parent' may become NULL in case of floating
containers. This commit checks the validity of parent.

fixes #821