support/scripts/size-stats: avoid divide-by-zero

Some packages (ex: skeleton-init-systemd) have a zero size so we cannot
divide by the package size. In that case make their percent zero
explicitly and avoid a ZeroDivisionError exception.

Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>