null: fix build with gcc 5.1

null: fix build with gcc 5.1

On Fedora 22, with GCC 5.1, errors are reported due to array accesses
being potentially out of bounds. This commit fixes this by adding in an
extra bounds check to the loop counters, or, in the case of stats reset,
by blindly zeroing the whole array, rather than just the part that is in
use.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>