MEDIUM: peers: unregister peers that were never started

The peers initialization sequence is a bit complex, they're attached
to stick-tables and initialized very early in the boot process. When
we fork, if some must not start, it's too late to find them. Instead,
simply add a guard in their respective tasks to stop them once they
want to start.
(cherry picked from commit 46dc1ca76114bff925460aee9439fc7dbef1185f)
diff --git a/src/peers.c b/src/peers.c
index 97cbcac..0564d3d 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1337,6 +1337,15 @@
 
 	task->expire = TICK_ETERNITY;
 
+	if (!st->sessions->peer->peers->peers_fe) {
+		/* this one was never started, kill it */
+		signal_unregister_handler(st->sighandler);
+		st->table->sync_task = NULL;
+		task_delete(st->sync_task);
+		task_free(st->sync_task);
+		return NULL;
+	}
+
 	if (!stopping) {
 		/* Normal case (not soft stop)*/
 		if (((st->flags & SHTABLE_RESYNC_STATEMASK) == SHTABLE_RESYNC_FROMLOCAL) &&