Do some minor pre cleanup work before converting all xfermodes to XPs.

src/effects/SkXfermodeImageFilter.cpp

Index: src/effects/SkXfermodeImageFilter.cpp
diff --git a/src/effects/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index 617593eae6e559b300a6f435f6f0bb0642b4a74f..14dde88379aa7dd8f990a79f1c85dbdb9192cb5c 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -127,6 +127,12 @@ bool SkXfermodeImageFilter::filterImageGPU(Proxy* proxy,
         return onFilterImage(proxy, src, ctx, result, offset);
     }
     GrTexture* backgroundTex = background.getTexture();
+
+    if (NULL == backgroundTex) {
+        SkASSERT(false);
+        return false;
+    }
+
     SkBitmap foreground = src;
     SkIPoint foregroundOffset = SkIPoint::Make(0, 0);
     if (getInput(1) && !getInput(1)->getInputResultGPU(proxy, src, ctx, &foreground,