[MINOR] silent gcc for a wrong warning

gcc believes that avoididx may be used uninitialized, which is wrong.
diff --git a/include/proto/backend.h b/include/proto/backend.h
index c4ffe85..1720105 100644
--- a/include/proto/backend.h
+++ b/include/proto/backend.h
@@ -66,6 +66,7 @@
 	newidx = px->lbprm.map.rr_idx;
 
 	avoided = NULL;
+	avoididx = 0; /* shut a gcc warning */
 	do {
 		srv = px->lbprm.map.srv[newidx++];
 		if (!srv->maxconn || srv->cur_sess < srv_dynamic_maxconn(srv)) {