[POWERPC] Fix possible division by zero in scaled time accounting

If we get no user time and no system time allocated since the last
account_system_vtime, the system to user time ratio estimate can end
up dividing by zero.

This was causing a problem noticed by Balbir Singh.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>