Avoid NaNs when node coincides with cluster centerpoint, per pull #1 from @nvie

Author: ericsoco

lib/forceCluster.js

@@ -27,7 +27,7 @@ function cluster (centers) {
       l = Math.sqrt(x * x + y * y),
       r = d.radius + (c.radius || 0);
 
-      if (l != r) {
+      if (l && l != r) {
         l = (l - r) / l * alpha;
         d.x -= x *= l;
         d.y -= y *= l;