In KUNIT_EXPECT_MEMEQ and KUNIT_EXPECT_MEMNEQ, add check if one of the
inputs is NULL and fail if this is the case.

Currently, the kernel crashes if one of the inputs is NULL. Instead,
fail the test and add an appropriate error message.

Fixes: b8a926be ("kunit: Introduce KUNIT_EXPECT_MEMEQ and KUNIT_EXPECT_MEMNEQ macros")

This was found by the kernel test robot:
https://lore.kernel.org/all/202212191448.D6EDPdOh-lkp@intel.com/Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>