Don't allow renderTarget==NULL to GrContext::clear() and friends.

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 #include "SkBitmap.h"
 #include "SkReadBuffer.h"
@@ skipping 276 matching lines @@
             context->refScratchTexture(maskDesc, GrContext::kApprox_ScratchTexMatch));
         if (!maskTexture) {
             return false;
         }
 
         {
             GrContext::AutoRenderTarget art(context, maskTexture->asRenderTarget());
             GrPaint grPaint;
             grPaint.setBlendFunc(kOne_GrBlendCoeff, kZero_GrBlendCoeff);
             SkRegion::Iterator iter(fRegion);
-            context->clear(NULL, 0x0, true);
+            context->clear(NULL, 0x0, true, maskTexture->asRenderTarget());
 
             SkMatrix old_matrix = context->getMatrix();
             context->setMatrix(in_matrix);
 
             while (!iter.done()) {
                 SkRect rect = SkRect::Make(iter.rect());
                 context->drawRect(grPaint, rect);
                 iter.next();
             }
             context->setMatrix(old_matrix);
@@ skipping 69 matching lines @@
                                                   (U8CPU)(SkColorGetG(source) * scale),
                                                   (U8CPU)(SkColorGetB(source) * scale));
                 }
             }
             dptr[y * dst->width() + x] = output_color;
         }
     }
 
     return true;
 }