BUG/MAJOR: threads/lb: fix missing unlock on consistent hash LB

If no matching node was found, the function was left without unlocking
the tree.
diff --git a/src/lb_chash.c b/src/lb_chash.c
index 32d7d1d..70a455d 100644
--- a/src/lb_chash.c
+++ b/src/lb_chash.c
@@ -388,9 +388,11 @@
 			node = eb32_first(root);
 
 		p->lbprm.chash.last = node;
-		if (!node)
+		if (!node) {
 			/* no node is available */
-			return NULL;
+			srv = NULL;
+			goto out;
+		}
 
 		/* Note: if we came here after a down/up cycle with no last
 		 * pointer, and after a redispatch (srvtoavoid is set), we