pstore/zone: Add a null pointer check to the psz_kmsg_read

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful
by checking the pointer validity.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240118100206.213928-1-chentao@kylinos.cn
Signed-off-by: Kees Cook <keescook@chromium.org>