examples/ioat: handle failure case for ioat dequeue

examples/ioat: handle failure case for ioat dequeue

On a failure with the ioat dequeue, -1 is returned, which was not properly
handled. This could lead to out-of-bounds reads on a later loop which
assumed a positive return value. Fix this by treating an error as though a
dequeue of 0 had been encountered.

Coverity issue: 350342
Coverity issue: 350349
Fixes: 2328542ed84e ("examples/ioat: add rawdev copy mode")
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>