Fix crash in ticks for empty d3.time.scale domain.

Fixes #1525. This was only an issue because the domain is extended by one
millisecond when it is empty (for an inclusive upper bound).

d3.js

@@ -8887,7 +8887,7 @@ d3 = function() {
         range: interval
       }, skip ];
       if (method) interval = method[0], skip = method[1];
-      return interval.range(extent[0], d3_time_scaleDate(+extent[1] + 1), skip);
+      return interval.range(extent[0], d3_time_scaleDate(+extent[1] + 1), skip < 1 ? 1 : skip);
     };
     scale.tickFormat = function() {
       return format;

src/time/scale.js

@@ -71,7 +71,7 @@ function d3_time_scale(linear, methods, format) {
 
     if (method) interval = method[0], skip = method[1];
 
-    return interval.range(extent[0], d3_time_scaleDate(+extent[1] + 1), skip); // inclusive upper bound
+    return interval.range(extent[0], d3_time_scaleDate(+extent[1] + 1), skip < 1 ? 1 : skip); // inclusive upper bound
   };
 
   scale.tickFormat = function() {

test/time/scale-test.js

@@ -313,6 +313,10 @@ suite.addBatch({
           local(1500, 0, 1, 0, 0),
           local(2000, 0, 1, 0, 0)
         ]);
+      },
+      "returns one tick for degenerate empty domain": function(scale) {
+        var x = scale().domain([local(2014, 2, 2), local(2014, 2, 2)]);
+        assert.deepEqual(x.ticks(6), [local(2014, 2, 2)]);
       }
     },
 
@@ -617,6 +621,10 @@ suite.addBatch({
           utc(1500, 0, 1, 0, 0),
           utc(2000, 0, 1, 0, 0)
         ]);
+      },
+      "returns one tick for degenerate empty domain": function(scale) {
+        var x = scale().domain([utc(2014, 2, 2), utc(2014, 2, 2)]);
+        assert.deepEqual(x.ticks(6), [utc(2014, 2, 2)]);
       }
     },