examples/ioat: handle error when querying number of stats

examples/ioat: handle error when querying number of stats

To get the amount of memory needed for stats, we call the xstats_get_names
function with a NULL parameter, which can return -1 on error. This negative
value was not previously handled correctly, so we adjust things to quit the
stats printing routine if this basic call fails.

Coverity issue: 350346
Fixes: 632bcd9b5d4f ("examples/ioat: print statistics")
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>