Do not allow benchmark mode to be used with scrypt.

cgminer.c

@@ -7477,6 +7477,8 @@ int main(int argc, char *argv[])
 	if (opt_benchmark) {
 		struct pool *pool;
 
+		if (opt_scrypt)
+			quit(1, "Cannot use benchmark mode with scrypt");
 		pool = add_pool();
 		pool->rpc_url = malloc(255);
 		strcpy(pool->rpc_url, "Benchmark");