Changes

coverity CID#27223

Make sure that bits_unused cannot subtract more bits than present in
st->size.

Especially when st->size == 0, this ensures that sizeinunits is also 0,
and that a st->size == 0 hence never enters the while (sizeinunits)
loop.

Change-Id: Ib4d4d08dde9dc01403a62493ab6ae8b853b8a1ec