src/pmdas/nvidia/nvidia.c: fix misuse of memset()
src/pmdas/nvidia/nvidia.c: fix misuse of memset()

Found by gcc on Ubuntu 17.10 ... the previous code was wrong and
if it worked, this was by accident.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp