don't create zero length intervals

gm/bug530095.cpp

Index: gm/bug530095.cpp
diff --git a/gm/bug530095.cpp b/gm/bug530095.cpp
index 690da3c9d0ba054b4f2094a522e9bdfffcfed264..76f6bc8f2e55f15cb80714ab2f2f02ac94f45548 100644
--- a/gm/bug530095.cpp
+++ b/gm/bug530095.cpp
@@ -46,3 +46,16 @@ DEF_SIMPLE_GM(bug530095, canvas, 900, 1200) {
     canvas->translate(4, 4);
     canvas->drawPath(path2, paint);
 }
+
+DEF_SIMPLE_GM(bug591993, canvas, 40, 140) {
+    SkPaint p;
+    p.setColor(SK_ColorRED);
+    p.setAntiAlias(true);
+    p.setStyle(SkPaint::kStroke_Style);
+    p.setStrokeCap(SkPaint::kRound_Cap);
+    p.setStrokeWidth(10);
+    SkScalar intervals[] = { 100, 100 };
+    SkPathEffect* dash = SkDashPathEffect::Create(intervals, SK_ARRAY_COUNT(intervals), 100);
+    p.setPathEffect(dash)->unref();
+    canvas->drawLine(20, 20, 120, 20, p);
+}