Adjust the alpha type for pixelRefs.

src/images/SkDecodingImageGenerator.cpp

Index: src/images/SkDecodingImageGenerator.cpp
diff --git a/src/images/SkDecodingImageGenerator.cpp b/src/images/SkDecodingImageGenerator.cpp
index 88cdef9a321f8a8875589aa7461e5a40611df57a..9f66011e9e233c87e5693217fa232c862f3e455f 100644
--- a/src/images/SkDecodingImageGenerator.cpp
+++ b/src/images/SkDecodingImageGenerator.cpp
@@ -246,6 +246,11 @@ SkImageGenerator* CreateDecodingImageGenerator(
     if (opts.fRequireUnpremul && info.fAlphaType != kOpaque_SkAlphaType) {
         info.fAlphaType = kUnpremul_SkAlphaType;
     }
+
+    if (!SkValidateAlphaType(info.fColorType, info.fAlphaType, &info.fAlphaType)) {
+        return NULL;
+    }
+
     return SkNEW_ARGS(DecodingImageGenerator,
                       (data, autoStream.detach(), info,
                        opts.fSampleSize, opts.fDitherImage));