stage: Add more checks on the stage window retrieval
stage: Add more checks on the stage window retrieval

During destruction, the StageWindow implementation associated to a Stage
might be NULL. We need to add more checks for a) the IN_DESTRUCTION flag
being set and b) the StageWindow pointer being NULL. Otherwise, we will
get warnings during the destruction of the Stage.

djdeath/clutter,collects/clutter,dlespiau/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,djdeath/clutter,nobled/clutter,djdeath/clutter-android,heysion/clutter-clone,jigpu/clutter,djdeath/clutter,djdeath/clutter-android,GNOME/clutter,nobled/clutter,djdeath/clutter-android,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter,GNOME/clutter,nobled/clutter,kerrickstaley/clutter-vala,ebassi/clutter,ebassi/clutter,Distrotech/clutter,ebassi/clutter,heysion/clutter-clone,jigpu/clutter,Distrotech/clutter,djdeath/clutter-android,GNOME/clutter,djdeath/clutter,djdeath/clutter-multithreaded,jigpu/clutter,dlespiau/clutter,ebassi/clutter,djdeath/clutter,ebassi/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,Distrotech/clutter,jigpu/clutter,jigpu/clutter,collects/clutter,collects/clutter,spatulasnout/clutter,djdeath/clutter-android,dlespiau/clutter,spatulasnout/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,GNOME/clutter,GNOME/clutter,spatulasnout/clutter,heysion/clutter-clone,Distrotech/clutter,Distrotech/clutter,collects/clutter,kerrickstaley/clutter-vala,jigpu/clutter,heysion/clutter-clone,collects/clutter,spatulasnout/clutter