lkdtm: fortify: Fix potential NULL dereference on kmalloc failure

Add missing NULL pointer checks after kmalloc() calls in
lkdtm_FORTIFY_STR_MEMBER() and lkdtm_FORTIFY_MEM_MEMBER() functions.

Signed-off-by: Junjie Cao <junjie.cao@intel.com>
Link: https://lore.kernel.org/r/20250814060605.5264-1-junjie.cao@intel.com


Signed-off-by: Kees Cook <kees@kernel.org>