cairo: apply patch for CVE-2018-19876

This applies the patch from the following upstream merge request:
    https://gitlab.freedesktop.org/cairo/cairo/merge_requests/5

This resulted in the following merge commit, which we fetch as a patch
and apply:
    https://gitlab.freedesktop.org/cairo/cairo/commit/6edf572ebb27b00d3c371ba5ae267e39d27d5b6d