more reinforcement that images never have zero width or height

src/image/SkImage_Gpu.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 71 matching lines @@
     return fBitmap.copyTo(dst, kN32_SkColorType);
 }
 
 bool SkImage_Gpu::isOpaque() const {
     return fBitmap.isOpaque();
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SkImage* SkNewImageFromBitmapTexture(const SkBitmap& bitmap, int sampleCountForNewSurfaces) {
-    if (NULL == bitmap.getTexture()) {
+    if (0 == bitmap.width() || 0 == bitmap.height() || NULL == bitmap.getTexture()) {
         return NULL;
     }
     return SkNEW_ARGS(SkImage_Gpu, (bitmap, sampleCountForNewSurfaces));
 }
 
 GrTexture* SkTextureImageGetTexture(SkImage* image) {
     return ((SkImage_Gpu*)image)->getTexture();
 }