[cairo] Fix for potential crash in pixman-compose.c

[Bertram Felgenhauer]

Antoine Azar wrote:
> here's a patch that fixes a potential crash in pixman-compose.c when a box 
> is used in some cases while not allocated or initialized. Mea culpa, the 
> bug was introduced in my refactoring of the fbFetchTransform.

I've pushed a simpler patch that Antoine, Soeren and I discussed
off list.

Bertram