Add test for new FrontBufferedStream behavior.

src/ports/SkImageDecoder_CG.cpp

Index: src/ports/SkImageDecoder_CG.cpp
diff --git a/src/ports/SkImageDecoder_CG.cpp b/src/ports/SkImageDecoder_CG.cpp
index 0358aec18c8c19b6c458c5e322f0a1cce5b0a851..73a95fc688e6a317a275e18e1f8d9c689f4a1b40 100644
--- a/src/ports/SkImageDecoder_CG.cpp
+++ b/src/ports/SkImageDecoder_CG.cpp
@@ -40,7 +40,9 @@ static CGDataProviderRef SkStreamToDataProvider(SkStream* stream) {
 
 static CGImageSourceRef SkStreamToCGImageSource(SkStream* stream) {
     CGDataProviderRef data = SkStreamToDataProvider(stream);
-    SkASSERT(data);
+    if (!data) {
+        return NULL;
+    }
     CGImageSourceRef imageSrc = CGImageSourceCreateWithDataProvider(data, 0);
     CGDataProviderRelease(data);
     return imageSrc;