Potentially fix segfaults on frontend startup

In nvctrl-util's GetNvidiaRates():
Initialise buf values to NULL, reduce it's size from 256 to 8 since
we're only interested in the first 8 tokens and actually check that we
parsed all 8 from the string before proceeding to use them.

While here change logging from VB_PLAYBACK to VB_GUI which makes more sense.

This may fix some frontend startup segfaults experienced by some
people since we ended up using an area of uninitialised memory in
strtol.

Coverity 746855
(cherry picked from commit 516719ad7e750a258bd2f72f79eae099f79d303b)