vertex-buffer: don't unref NULL object
vertex-buffer: don't unref NULL object

In cogl_vertex_buffer_indices_get_for_quads() we sometimes have to
extend the length of an existing array, but when we came to unref the
previous array we didn't first check that it wasn't simply NULL.

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