lib: avoid crash when cancelling invalid rw job

lib: avoid crash when cancelling invalid rw job

There are some observed instances where we end up trying to cancel a rw
job based on a file descriptor that we don't have a reference on. The
specific cancel function for rw jobs assumes it's called with a file
descriptor that is valid within pollfds and will cause a segmentation
fault by buffer overrun if this is not the case.

Instead log it and move on. Since the fd does not exist this should
patch over the buggy behavior and provide additional information to help
in finding the root cause.

Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>