Handle NULL typeface in SampleApp.

It is possible that SampleApp will use the NULL typeface as the default.
This is valid and should be handled properly on shutdown.
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 8825f53..37377ae 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -973,8 +973,7 @@
 
 SampleWindow::~SampleWindow() {
     delete fPdfCanvas;
-    fTypeface->unref();
-
+    SkSafeUnref(fTypeface);
     SkSafeUnref(fDevManager);
 }