Merge branch 'force_layout_divide_by_zero' of https://github.com/Rhizi/d3 into fix-force-weight-zero

updated upstream into a topic branch.

d3.js

@@ -6268,7 +6268,7 @@
           l = alpha * strengths[i] * ((l = Math.sqrt(l)) - distances[i]) / l;
           x *= l;
           y *= l;
-          t.x -= x * (k = s.weight / (t.weight + s.weight));
+          t.x -= x * (k = s.weight + t.weight ? s.weight / (s.weight + t.weight) : .5);
           t.y -= y * k;
           s.x += x * (k = 1 - k);
           s.y += y * k;

src/layout/force.js

@@ -87,7 +87,7 @@ d3.layout.force = function() {
         l = alpha * strengths[i] * ((l = Math.sqrt(l)) - distances[i]) / l;
         x *= l;
         y *= l;
-        t.x -= x * (k = s.weight / (t.weight + s.weight));
+        t.x -= x * (k = (s.weight + t.weight ? s.weight / (s.weight + t.weight) : .5));
         t.y -= y * k;
         s.x += x * (k = 1 - k);
         s.y += y * k;