Fix potential integer overflow in Balance (#40)

An usize overflow can occur in `Balance::promote_to_ready` when `self.not_ready` has length 0.

See my comment here: https://github.com/tower-rs/tower/pull/39#discussion_r163967979

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tower-balance/src/lib.rs

@@ -148,6 +148,11 @@ where
         -> Result<(), Error<<D::Service as Service>::Error, D::DiscoverError>>
     {
         let n = self.not_ready.len();
+        if n == 0 {
+            trace!("promoting to ready: not_ready is empty, skipping.");
+            return Ok(());
+        }
+
         debug!("promoting to ready: {}", n);
         // Iterate through the not-ready endpoints from right to left to prevent removals
         // from reordering services in a way that could prevent a service from being polled.