more reinforcement that images never have zero width or height

src/image/SkImage_Gpu.cpp

Index: src/image/SkImage_Gpu.cpp
diff --git a/src/image/SkImage_Gpu.cpp b/src/image/SkImage_Gpu.cpp
index 350a1e88dec2ca37fafaa75463c27d72b7dc0272..8169b2a131b90f88911beccbffcbf6f6bc060f31 100644
--- a/src/image/SkImage_Gpu.cpp
+++ b/src/image/SkImage_Gpu.cpp
@@ -89,7 +89,7 @@ bool SkImage_Gpu::isOpaque() const {
 ///////////////////////////////////////////////////////////////////////////////
 
 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));