Revert "Initialise all the pointers in GetNvidiaRates()'s buf[256] to
NULL. Coverity 746855"

This is causing a segfault at frontend startup for me, switching it to
use memset() instead causes the same problem. I suspect that 256 is
far larger than it needs to be but I'm not sure whether that's causing
us to run out of memory?

This reverts commit f1d33c50ed09ac1b151a0bac1c5b09b386501313.