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.