zfs: Fix malloc() success check

This code was hitting the error code path whenever malloc() succeeded
rather than when it failed, so presumably this part of the code hasn't
been tested. I had to apply this fix (and others) to get U-Boot to boot
from ZFS on an Nvidia Jetson TX2 NX SoM (an aarch64 computer).

Signed-off-by: Phaedrus Leeds <mwleeds@mailtundra.com>