blkiomon: Fix an output error

blkiomon: Fix an output error

When we give out some statistics in blkiomon, we don't consider
the situation that the device has no correspoinding action. See
if there is no disk read during the interval, the output in my box is
like:
sizes read (bytes): num 0, min -1, max 0, sum 0, squ 0, avg nan, var nan

With the fix, now it looks like:
sizes read (bytes): num 0, min -1, max 0, sum 0, squ 0, avg 0.0, var 0.0

Cc: Martin Peschke <mpeschke@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>