AcpiOsFree forwards NULL to _kfree which panics ("trying to free NULL pointer") — latent NULL-free panic unique to the DragonFly port (Linux/FreeBSD kfree(NULL) are no-ops)

Summary

The OSL was ported onto an allocator with different NULL semantics; all current callers guard, so latent - any future ACPICA sync or driver error path that frees an unallocated buffer becomes a panic. One-line guard fix.