BUILD: connections: shut up gcc about impossible out-of-bounds warning

Since commit 88698d9 ("MEDIUM: connections: Add a way to control the
number of idling connections.") when building without threads, gcc
complains that the operations made on the idle_orphan_conns[] list is
out of bounds, which is always false since 1) <i> can only equal zero,
and 2) given it's equal to <tid> we never even enter the loop. But as
usual it thinks it knows better, so let's mask the origin of this <i>
value to shut it up. Another solution consists in making <i> unsigned
and adding an explicit range check.

src/backend.c

@@ -1354,6 +1354,13 @@ int connect_server(struct stream *s)
 			for (i = 0; i < global.nbthread; i++) {
 				if (i == tid)
 					continue;
+
+				// just silence stupid gcc which reports an absurd
+				// out-of-bounds warning for <i> which is always
+				// exactly zero without threads, but it seems to
+				// see it possibly larger.
+				ALREADY_CHECKED(i);
+
 				tokill_conn = LIST_POP_LOCKED(&srv->idle_orphan_conns[i],
 				    struct connection *, list);
 				if (tokill_conn) {