make allocPixels throw on failure

src/image/SkImage.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 "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkImagePriv.h"
@@ skipping 99 matching lines @@
 bool SkImage_Base::onReadPixels(SkBitmap* bitmap, const SkIRect& subset) const {
     if (bitmap->pixelRef()) {
         const SkImageInfo info = bitmap->info();
         if (kUnknown_SkColorType == info.colorType()) {
             return false;
         }
         if (!raster_canvas_supports(info)) {
             return false;
         }
     } else {
-        const SkImageInfo info = SkImageInfo::MakeN32Premul(subset.width(), subset.height());
         SkBitmap tmp;
-        if (!tmp.allocPixels(info)) {
+        if (!tmp.tryAllocN32Pixels(subset.width(), subset.height())) {
             return false;
         }
         *bitmap = tmp;
     }
 
     SkRect srcR, dstR;
     srcR.set(subset);
     dstR = srcR;
     dstR.offset(-dstR.left(), -dstR.top());
 
     SkCanvas canvas(*bitmap);
 
     SkPaint paint;
     paint.setXfermodeMode(SkXfermode::kClear_Mode);
     canvas.drawRect(dstR, paint);
 
     const_cast<SkImage_Base*>(this)->onDrawRectToRect(&canvas, &srcR, dstR, NULL);
     return true;
 }