[cairo] Copy of path in cairo 1.4.10

[Chris Wilson]

Fred Kiefer (fredkiefer at gmx.de) said: 
> Perhaps it would be possible to come up with a special error value for
> this case or even not report it as an error.

Indeed there is no reason to raise an error here, so lets just return an
empty path.

I've pushed a fix, commit b4f86638cc4b87bfaf10568ae9beb89626e26613, to
master, but maybe it would also be suitable for a 1.4.x release?
--
Chris Wilson