Handle SkBitmaps which have no pixels in SkPDFImage.

src/pdf/SkPDFImage.cpp

Index: src/pdf/SkPDFImage.cpp
diff --git a/src/pdf/SkPDFImage.cpp b/src/pdf/SkPDFImage.cpp
index 122d2f214845dfcc111bdbe262b020ec219eeba5..73d85816aa7e070b4d022dcfe76af218597cf81a 100644
--- a/src/pdf/SkPDFImage.cpp
+++ b/src/pdf/SkPDFImage.cpp
@@ -222,7 +222,11 @@ static SkStream* extract_image_data(const SkBitmap& bitmap,
     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) {
@@ -253,7 +257,6 @@ static SkStream* extract_image_data(const SkBitmap& bitmap,
         default:
             SkASSERT(false);
     }
-    bitmap.unlockPixels();
 
     if (isTransparent != NULL) {
         *isTransparent = transparent;