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/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 8ae7646..fc5653b 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -550,7 +550,7 @@
     SkASSERT(NULL != context);
     SkMatrix matrix;
     if (!this->getLocalMatrix().invert(&matrix)) {
-        return false;
+        return NULL;
     }
     matrix.postConcat(fPtsToUnit);
     return SkNEW_ARGS(GrLinearGradient, (context, *this, matrix, fTileMode));