BUG/MINOR: connection: fix loop iter on connection takeover

The loop always missed one iteration due to the incrementation done on
the for check. Move the incrementation on the loop last statement to fix
this behaviour.

This bug has a very limited impact, not at all visible to the user, but
could be backported to 2.2.
diff --git a/src/backend.c b/src/backend.c
index 490dc1f..19bce9c 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1154,7 +1154,8 @@
 	if (stop >= global.nbthread)
 		stop = 0;
 
-	for (i = stop; !found && (i = ((i + 1 == global.nbthread) ? 0 : i + 1)) != stop;) {
+	i = stop;
+	do {
 		struct mt_list *elt1, elt2;
 
 		if (!srv->curr_idle_thr[i] || i == tid)
@@ -1183,7 +1184,7 @@
 			}
 		}
 		HA_SPIN_UNLOCK(OTHER_LOCK, &idle_conns[i].takeover_lock);
-	}
+	} while (!found && (i = (i + 1 == global.nbthread) ? 0 : i + 1) != stop);
 
 	if (!found)
 		conn = NULL;