PM / devfreq: Check governor before using governor->name

In the Linux kernel, the following vulnerability has been resolved: PM / devfreq: Check governor before using governor->name Commit 96ffcdf239de ("PM / devfreq: Remove redundant governor_name from struct devfreq") removes governor_name and uses governor->name to replace it. But devfreq->governor may be NULL and directly using devfreq->governor->name may cause null pointer exception. Move the check of governor to before using governor->name.