lib: packing: catch kunit_kzalloc() failure in the pack() test

lib: packing: catch kunit_kzalloc() failure in the pack() test

kunit_kzalloc() may fail. Other call sites verify that this is the case,
either using a direct comparison with the NULL pointer, or the
KUNIT_ASSERT_NOT_NULL() or KUNIT_ASSERT_NOT_ERR_OR_NULL().

Pick KUNIT_ASSERT_NOT_NULL() as the error handling method that made most
sense to me. It's an unlikely thing to happen, but at least we call
__kunit_abort() instead of dereferencing this NULL pointer.

Fixes: e9502ea6db8a ("lib: packing: add KUnit tests adapted from selftests")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241004110012.1323427-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>