pdfviewer: debug code for crash

Review URL: https://codereview.chromium.org/22269002

git-svn-id: http://skia.googlecode.com/svn/trunk@10548 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/experimental/PdfViewer/SkPdfRenderer.cpp b/experimental/PdfViewer/SkPdfRenderer.cpp
index 50b48a0..bb2049e 100644
--- a/experimental/PdfViewer/SkPdfRenderer.cpp
+++ b/experimental/PdfViewer/SkPdfRenderer.cpp
@@ -265,7 +265,7 @@
             const SkClipStack::Element* elem;
             double y = 0;
             int total = 0;
-            while (elem = iter.next()) {
+            while ((elem = iter.next()) != NULL) {
                 total++;
                 y += 30;