cashew: fix NDEBUG build failure

DCHECK_NOTNULL(foo) produces a "statement has no effect" error during
NDEBUG compilation, while DCHECK(foo != NULL) does not.

BUG=chromium-os:8800
TEST=USE=-cros-debug emerge-<board> cashew

Change-Id: I6e62d0e2f90f1412cd2d252a11e73623c308178d

TBR=djkurtz@chromium.org