nvme/linux: check for empty digest in gen_tls_identity()

Newer GCC complain about 'digest' being an empty argument, which
is technically true, but in practice can only happen if version == 0,
which we already check. So add a warning to keep GCC happy.

Signed-off-by: Hannes Reinecke <[email protected]>

Author: hreinecke

src/nvme/linux.c

@@ -1275,7 +1275,7 @@ static int gen_tls_identity(const char *hostnqn, const char *subsysnqn,
 			version, cipher, hostnqn, subsysnqn);
 		return strlen(identity);
 	}
-	if (version > 1) {
+	if (version > 1 || !digest) {
 		errno = EINVAL;
 		return -1;
 	}