more reinforcement that images never have zero width or height

src/image/SkImage_Raster.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImage_Base.h"
 #include "SkImagePriv.h"
 #include "SkBitmap.h"
@@ skipping 168 matching lines @@
     }
 
     return SkNEW_ARGS(SkImage_Raster, (info, data, rowBytes, NULL));
 }
 
 SkImage* SkImage::NewFromGenerator(SkImageGenerator* generator) {
     SkBitmap bitmap;
     if (!SkInstallDiscardablePixelRef(generator, &bitmap)) {
         return NULL;
     }
+    if (0 == bitmap.width() || 0 == bitmap.height()) {
+        return NULL;
+    }
+
     return SkNEW_ARGS(SkImage_Raster, (bitmap, NULL));
 }
 
 SkImage* SkNewImageFromPixelRef(const SkImageInfo& info, SkPixelRef* pr, size_t rowBytes,
                                 const SkSurfaceProps* props) {
+    if (!SkImage_Raster::ValidArgs(info, rowBytes)) {
+        return NULL;
+    }
     return SkNEW_ARGS(SkImage_Raster, (info, pr, rowBytes, props));
 }
 
 const SkPixelRef* SkBitmapImageGetPixelRef(const SkImage* image) {
     return ((const SkImage_Raster*)image)->getPixelRef();
 }
 
 bool SkImage_Raster::isOpaque() const {
     return fBitmap.isOpaque();
 }