[process] Make it safe to call process_add() multiple times

src/net/retry.c

@@ -187,5 +187,6 @@ static void retry_step ( struct process *process __unused ) {
 
 /** Retry timer process */
 struct process retry_process __permanent_process = {
+	.list = LIST_HEAD_INIT ( retry_process.list ),
 	.step = retry_step,
 };