If "ur_idx" is wrong we could go past the end of the array.  The
"ur_idx" comes from root so it's not a huge deal, but adding a sanity
check makes the code more robust.
Signed-off-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>