Avoid building recursive entities

For https://bugzilla.gnome.org/show_bug.cgi?id=762100

When we detect a recusive entity we should really not
build the associated data, moreover if someone bypass
libxml2 fatal errors and still tries to serialize a broken
entity make sure we don't risk to get ito a recursion

* parser.c: xmlParserEntityCheck() don't build if entity loop
  were found and remove the associated text content
* tree.c: xmlStringGetNodeList() avoid a potential recursion