pybootchart/draw: Avoid divide by zero error

When disk stats don't run frequenctly enough, we see divide by zero
errors. The code already has a fallback path so ensure we use it
for this case too.

[YOCTO #14360]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b71d30ae)
Signed-off-by: Steve Sakoman <steve@sakoman.com>