bug 24442 and bug 24524. The content stack wasn't addref'ing the content objects pushed
onto it while the content sink destructor was releasing the content objects. This caused
a crash because the root content object would get freed one too many times. Now,
PushContent() addrefs the content object before inserting it into the content stack.
git-svn-id: svn://10.0.0.236/trunk@58332 18797224-902f-48f8-a5cc-f745e15eee43