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

src/effects/SkXfermodeImageFilter.cpp

 /*
  * Copyright 2013 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkXfermodeImageFilter.h"
 #include "SkCanvas.h"
 #include "SkDevice.h"
@@ skipping 109 matching lines @@
                                            const Context& ctx,
                                            SkBitmap* result,
                                            SkIPoint* offset) const {
     SkBitmap background = src;
     SkIPoint backgroundOffset = SkIPoint::Make(0, 0);
     if (getInput(0) && !getInput(0)->getInputResultGPU(proxy, src, ctx, &background,
                                                        &backgroundOffset)) {
         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,
                                                        &foregroundOffset)) {
         return onFilterImage(proxy, src, ctx, result, offset);
     }
     GrTexture* foregroundTex = foreground.getTexture();
     GrContext* context = foregroundTex->getContext();
 
     GrFragmentProcessor* xferProcessor = NULL;
@@ skipping 30 matching lines @@
     context->drawRect(paint, SkMatrix::I(), srcRect);
 
     offset->fX = backgroundOffset.fX;
     offset->fY = backgroundOffset.fY;
     WrapTexture(dst, src.width(), src.height(), result);
     return true;
 }
 
 #endif