Fix assert when dumping view at 0 alpha

bug:21461177

Change-Id: Ib4aba7b9641254012471da44aa804813b9679423
diff --git a/libs/hwui/RenderNode.cpp b/libs/hwui/RenderNode.cpp
index 9146b68..9e5ec28 100644
--- a/libs/hwui/RenderNode.cpp
+++ b/libs/hwui/RenderNode.cpp
@@ -83,8 +83,9 @@
  * display list. This function should remain in sync with the replay() function.
  */
 void RenderNode::output(uint32_t level) {
-    ALOGD("%*sStart display list (%p, %s%s%s%s)", (level - 1) * 2, "", this,
+    ALOGD("%*sStart display list (%p, %s%s%s%s%s)", (level - 1) * 2, "", this,
             getName(),
+            (MathUtils::isZero(properties().getAlpha()) ? ", zero alpha" : ""),
             (properties().hasShadow() ? ", casting shadow" : ""),
             (isRenderable() ? "" : ", empty"),
             (mLayer != nullptr ? ", on HW Layer" : ""));