More robust handling of polygons with empty ring(s).

This is a valid situation but won't usually happen at the moment since
we don't return empty rings when clipping.

d3.js

@@ -6144,7 +6144,7 @@
         var n = coordinates.length, i = -1, m, j, ring, point;
         while (++i < n) {
           ring = coordinates[i];
-          if (!(m = ring.length - 1)) continue;
+          if ((m = ring.length - 1) < 1) continue;
           point = ring[0];
           context.moveTo(point[0], point[1]);
           for (j = 0; ++j < m; ) {

src/geo/path.js

@@ -40,7 +40,7 @@ d3.geo.path = function() {
           point;
       while (++i < n) {
         ring = coordinates[i];
-        if (!(m = ring.length - 1)) continue;
+        if ((m = ring.length - 1) < 1) continue;
         point = ring[0];
         context.moveTo(point[0], point[1]);
         for (j = 0; ++j < m;) {

test/geo/path-test.js

@@ -93,6 +93,7 @@ suite.addBatch({
       assert.isNull(path(undefined));
       assert.isNull(path());
     },
+
     "bogus type name": function(path) {
       assert.isNull(path({
         type: "Feature",