Improve the safety checks inside the text setters
Improve the safety checks inside the text setters

We should check that the contents of the Text actor are
not NULL when computing the offsets in bytes.

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