Use Shadow DOM to display fallback content for images

Source/platform/image-decoders/png/PNGImageDecoder.cpp

Index: Source/platform/image-decoders/png/PNGImageDecoder.cpp
diff --git a/Source/platform/image-decoders/png/PNGImageDecoder.cpp b/Source/platform/image-decoders/png/PNGImageDecoder.cpp
index 86cff4c7c37adf2e9c959417eee6c65c3cd0013a..70b5315e69945a0dcff5a444ec3fceb027f90987 100644
--- a/Source/platform/image-decoders/png/PNGImageDecoder.cpp
+++ b/Source/platform/image-decoders/png/PNGImageDecoder.cpp
@@ -142,8 +142,11 @@ public:
         PNGImageDecoder* decoder = static_cast<PNGImageDecoder*>(png_get_progressive_ptr(m_png));
 
         // We need to do the setjmp here. Otherwise bad things will happen.
+        // The libpng manual points out: ".. there are some uncertainties about the status of local variables after a longjmp, so the
+        // user may want to be careful about doing anything after setjmp returns non-zero besides returning itself." This is why
+        // we avoid referencing |decoder| in the event of an error.
         if (setjmp(JMPBUF(m_png)))
-            return decoder->setFailed();
+            return static_cast<PNGImageDecoder*>(png_get_progressive_ptr(m_png))->setFailed();
 
         const char* segment;
         while (unsigned segmentLength = data.getSomeData(segment, m_readOffset)) {