CVE-2025-40129 sunrpc: fix null pointer dereference on zero-length checksum

In the Linux kernel, the following vulnerability has been resolved: sunrpc: fix null pointer dereference on zero-length checksum In xdrstreamdecodeopaqueauth, zero-length checksum.len causes checksum.data to be set to NULL. This triggers a NPD when accessing checksum.data in gsskrb5verifymicv2...