allow one zero length dash

gm/dashing.cpp

Index: gm/dashing.cpp
diff --git a/gm/dashing.cpp b/gm/dashing.cpp
index 0e249c7d61cae7248206b857e2d31d63e12d41df..f69a37ecd117e34167fefc9ba5ec76c937da84ef 100644
--- a/gm/dashing.cpp
+++ b/gm/dashing.cpp
@@ -21,7 +21,8 @@ static void drawline(SkCanvas* canvas, int on, int off, const SkPaint& paint,
         SkIntToScalar(off),
     };
 
-    p.setPathEffect(SkDashPathEffect::Create(intervals, 2, phase))->unref();
+    SkAutoTUnref<SkPathEffect> effect(SkDashPathEffect::Create(intervals, 2, phase));
+    p.setPathEffect(effect);
     canvas->drawLine(startX, startY, finalX, finalY, p);
 }