Add a fix for a spurious assert in SkMatrixConvolutionImageFilter.

src/effects/SkMatrixConvolutionImageFilter.cpp

 /*
  * Copyright 2012 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 "SkMatrixConvolutionImageFilter.h"
 #include "SkBitmap.h"
 #include "SkColorPriv.h"
@@ skipping 135 matching lines @@
             return *src.getAddr32(x, y);
         }
     }
 };
 
 template<class PixelFetcher, bool convolveAlpha>
 void SkMatrixConvolutionImageFilter::filterPixels(const SkBitmap& src,
                                                   SkBitmap* result,
                                                   const SkIRect& rect,
                                                   const SkIRect& bounds) const {
+    if (rect.isEmpty()) {
+        return;
+    }
     for (int y = rect.fTop; y < rect.fBottom; ++y) {
         SkPMColor* dptr = result->getAddr32(rect.fLeft - bounds.fLeft, y - bounds.fTop);
         for (int x = rect.fLeft; x < rect.fRight; ++x) {
             SkScalar sumA = 0, sumR = 0, sumG = 0, sumB = 0;
             for (int cy = 0; cy < fKernelSize.fHeight; cy++) {
                 for (int cx = 0; cx < fKernelSize.fWidth; cx++) {
                     SkPMColor s = PixelFetcher::fetch(src,
                                                       x + cx - fKernelOffset.fX,
                                                       y + cy - fKernelOffset.fY,
                                                       bounds);
@@ skipping 509 matching lines @@
                                                 fBias,
                                                 fKernelOffset,
                                                 fTileMode,
                                                 fConvolveAlpha);
     return true;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 #endif