Destroy SkScanlineDecoder in the SkCodec subclass destructors

src/codec/SkCodec_libpng.cpp

Index: src/codec/SkCodec_libpng.cpp
diff --git a/src/codec/SkCodec_libpng.cpp b/src/codec/SkCodec_libpng.cpp
index 444e2ad87e0ae14a3bfd95746fd991f592c7ceca..c8d331db52bae2d4931e334b034b72d34135a49f 100644
--- a/src/codec/SkCodec_libpng.cpp
+++ b/src/codec/SkCodec_libpng.cpp
@@ -514,6 +514,12 @@ bool SkPngCodec::handleRewind() {
 SkCodec::Result SkPngCodec::onGetPixels(const SkImageInfo& requestedInfo, void* dst,
                                         size_t rowBytes, const Options& options,
                                         SkPMColor ctable[], int* ctableCount) {
+    // Do not allow a regular decode if the caller has asked for a scanline decoder
+    if (NULL != this->scanlineDecoder()) {
+        SkCodecPrintf("cannot getPixels() if a scanline decoder has been created\n");
+        return kInvalidInput;

[scroggo, 2015/06/30 14:50:17]

This is intended to mean that the encoded data is invalid. In this case, it was
an incorrect use by the caller. SkImageGenerator::kInvalidParameters seems like
the error code that most closely matches the error (or maybe it should be a new
code?).

[msarett, 2015/06/30 20:43:05]

I agree, InvalidParameters makes more sense.

+    }
+
     if (!this->handleRewind()) {
         return kCouldNotRewind;
     }
@@ -734,7 +740,7 @@ private:
     
     
     
-    
+
 
     typedef SkScanlineDecoder INHERITED;
 };