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