Make linux sample app work on my laptop. Reviewed at http://codereview.appspot.com/4568050/

git-svn-id: http://skia.googlecode.com/svn/trunk@1539 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 3efffe6..f05fe6e 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -848,7 +848,7 @@
             break;
 #ifdef SK_SUPPORT_GL
         case kGPU_CanvasType:
-            if (fShowZoomer) {
+            if (fShowZoomer && fGpuCanvas) {
                 this->showZoomer(fGpuCanvas);
             }
             delete fGpuCanvas;