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 xdr_stream_decode_opaque_auth(), zero-length checksum.len causes checksum.data to be set to NULL. This triggers a NPD when accessing checksum.data in gss_krb5_verify_mic_v2(). This patch ensures that the value of checksum.len is not less than XDR_UNIT.