g_cancellable_get_fd: silently return -1 for NULL cancellable
g_cancellable_get_fd: silently return -1 for NULL cancellable

This keeps compatibility with previous behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=655598

Signed-off-by: David Zeuthen <16a4bae6a7afea1fdd75baca6404fe68f285db96@redhat.com>

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