Check IsDetached() in ImageDocumentParser::AppendBytes()

It is suspicious that GetDocument() can be nullptr in
ImageDocumentParser::AppendBytes().
This CL adds an IsDetached() check there, just like
DecodedDataDocumentParser::AppendBytes() does.

Bug: 826198
Change-Id: I297fe5bbe609a380ce8a175529d1216a88d1c1ee
Reviewed-on: https://chromium-review.googlesource.com/1050697Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557244}

third_party/blink/renderer/core/html/image_document.cc

@@ -122,6 +122,9 @@ void ImageDocumentParser::AppendBytes(const char* data, size_t length) {
   if (!length)
     return;
 
+  if (IsDetached())
+    return;
+
   LocalFrame* frame = GetDocument()->GetFrame();
   Settings* settings = frame->GetSettings();
   if (!frame->GetContentSettingsClient()->AllowImage(