Handle SkBitmaps which have no pixels in SkPDFImage.

src/pdf/SkPDFImage.cpp

 /*
  * Copyright 2010 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 "SkPDFImage.h"
 
 #include "SkBitmap.h"
@@ skipping 204 matching lines @@
                          kRGB_565_SkColorType == colorType)) {
         if (isTransparent != NULL) {
             *isTransparent = false;
         }
         return NULL;
     }
     bool isOpaque = true;
     bool transparent = extractAlpha;
     SkStream* stream = NULL;
 
-    bitmap.lockPixels();
+    SkAutoLockPixels lock(bitmap);
+    if (NULL == bitmap.getPixels()) {
+        return NULL;
+    }
+
     switch (colorType) {
         case kIndex_8_SkColorType:
             if (!extractAlpha) {
                 stream = extract_index8_image(bitmap, srcRect);
             }
             break;
         case kARGB_4444_SkColorType:
             stream = extract_argb4444_data(bitmap, srcRect, extractAlpha,
                                            &isOpaque, &transparent);
             break;
@@ skipping 10 matching lines @@
             if (!extractAlpha) {
                 stream = create_black_image();
             } else {
                 stream = extract_a8_alpha(bitmap, srcRect,
                                           &isOpaque, &transparent);
             }
             break;
         default:
             SkASSERT(false);
     }
-    bitmap.unlockPixels();
 
     if (isTransparent != NULL) {
         *isTransparent = transparent;
     }
     if (extractAlpha && (transparent || isOpaque)) {
         SkSafeUnref(stream);
         return NULL;
     }
     return stream;
 }
@@ skipping 446 matching lines @@
     if (SkIRect::MakeWH(bitmap.width(), bitmap.height()) == subset) {
         SkAutoTUnref<SkData> encodedData(ref_encoded_data(bitmap));
         if (is_jfif_jpeg(encodedData)) {
             return SkNEW_ARGS(PDFJPEGImage,
                               (encodedData, bitmap.width(), bitmap.height()));
         }
     }
 #endif
     return SkPDFImage::CreateImage(bitmap, subset, encoder);
 }