...but not when it's being used as a digest algorithm...

src/crypto/crypto_null.c

 
 static void null_encode ( void *ctx __unused, const void *src,
 			  void *dst, size_t len ) {
-	memcpy ( dst, src, len );
+	if ( dst )
+		memcpy ( dst, src, len );
 }
 
 static void null_decode ( void *ctx __unused, const void *src,
 			  void *dst, size_t len ) {
-	memcpy ( dst, src, len );
+	if ( dst )
+		memcpy ( dst, src, len );
 }
 
 static void null_final ( void *ctx __unused, void *out __unused ) {