Bug 1350049: Handle nsParser being finalized before resuming. r=hsivonen

This is a hybrid of the previous two approaches. The nsParser weak reference
sometimes stays alive after it's been detached from the document, after which
attempting to remove it throws. This stores a reference to the original
parser, and checks that it's still the current parser when it comes time to
resume.

MozReview-Commit-ID: 1JSi2FmPxt0

parser/htmlparser/nsParser.cpp

@@ -682,7 +682,10 @@ nsParser::UnblockParser()
 NS_IMETHODIMP_(void)
 nsParser::ContinueInterruptedParsingAsync()
 {
-  mSink->ContinueInterruptedParsingAsync();
+  MOZ_ASSERT(mSink);
+  if (MOZ_LIKELY(mSink)) {
+    mSink->ContinueInterruptedParsingAsync();
+  }
 }
 
 /**