fix warning from scalar --> int32 conversion

BUG=

Review URL: https://codereview.appspot.com/7065050

git-svn-id: http://skia.googlecode.com/svn/trunk@7061 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/WritePixelsTest.cpp b/tests/WritePixelsTest.cpp
index f5c4175..715d8f7 100644
--- a/tests/WritePixelsTest.cpp
+++ b/tests/WritePixelsTest.cpp
@@ -136,7 +136,7 @@
     static SkBitmap bmp;
     if (bmp.isNull()) {
         bmp.setConfig(SkBitmap::kARGB_8888_Config, DEV_W, DEV_H);
-        bool alloc = bmp.allocPixels();
+        SkDEBUGCODE(bool alloc = ) bmp.allocPixels();
         SkASSERT(alloc);
         SkAutoLockPixels alp(bmp);
         intptr_t pixels = reinterpret_cast<intptr_t>(bmp.getPixels());