another false --> NULL return fix

false --> NULL for failure return

BUG=

Review URL: https://codereview.appspot.com/7100052

git-svn-id: http://skia.googlecode.com/svn/trunk@7167 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index 574dfe3..41de802 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -174,7 +174,7 @@
 }
 
 GrEffect* SkShader::asNewEffect(GrContext*, const SkPaint&) const {
-    return false;
+    return NULL;
 }
 
 SkShader* SkShader::CreateBitmapShader(const SkBitmap& src,