make allocPixels throw on failure

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 01b7c69c257522586987487b51443787a2b40b7a..b69f87996c76c5e5a96fa7eac063d62d1f4aab02 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -823,7 +823,7 @@ bool SkPNGImageDecoder::onDecodeSubset(SkBitmap* bm, const SkIRect& region) {
             return false;
         }
     } else {
-        if (!decodedBitmap.allocPixels(NULL, needColorTable ? colorTable : NULL)) {
+        if (!decodedBitmap.tryAllocPixels(NULL, needColorTable ? colorTable : NULL)) {
             return false;
         }
     }