kselftest: arm64: Add a null pointer check

kselftest: arm64: Add a null pointer check

There is a 'malloc' call, which can be unsuccessful.
This patch will add the malloc failure checking
to avoid possible null dereference and give more information
about test fail reasons.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240423082102.2018886-1-chentao@kylinos.cn
Signed-off-by: Will Deacon <will@kernel.org>