Changes

coverity CID#57681

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: Ic8e3f5e24541989b79826de2942e5e6560079028