fix all the false --> NULL issues in the gradient code

BUG=

Review URL: https://codereview.appspot.com/7095056

git-svn-id: http://skia.googlecode.com/svn/trunk@7170 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/effects/gradients/SkTwoPointConicalGradient.cpp b/src/effects/gradients/SkTwoPointConicalGradient.cpp
index 6fab6d4..37b8a51 100644
--- a/src/effects/gradients/SkTwoPointConicalGradient.cpp
+++ b/src/effects/gradients/SkTwoPointConicalGradient.cpp
@@ -690,7 +690,7 @@
     // invert the localM, translate to center1, rotate so center2 is on x axis.
     SkMatrix matrix;
     if (!this->getLocalMatrix().invert(&matrix)) {
-        return false;
+        return NULL;
     }
     matrix.postTranslate(-fCenter1.fX, -fCenter1.fY);