cpupower: Fix sscanf robustness in cpufreq-set

The cpufreq-set tool has a missing length check. This is basically
just correctness but still should get fixed.

One of a set of sscanf problems reported by Jackie Chang

Signed-off-by: Alan Cox <alan@linux.intel.com>
[rjw: Subject]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>