Fix ownership of xmlNodePtr & xmlAttrPtr fields in xmlSetTreeDoc()

When changing `doc` on an xmlNodePtr or xmlAttrPtr, certain
fields must either be a free-standing string, or they must be
owned by `doc->dict`.

The code to make this change was simply missing, so the crash
happened when an xmlAttrPtr was being torn down after `doc`
changed from non-NULL to NULL, but the `name` field was not
copied.  This is scenario 1 below.

The xmlNodePtr->name and xmlNodePtr->content fields are also
fixed at the same time.  Note that xmlNodePtr->content is never
added to the dictionary, so NULL is used instead of `newDict` to
force a free-standing copy.

This change covers all cases of dictionary changes:
1. Owned by old dictionary -> NULL new dictionary
   - Create free-standing copy of string.
2. Owned by old dictionary -> Non-NULL new dictionary
   - Get string from new dictionary pool.
3. Not owned by old dictionary -> Non-NULL new dictionary
   - No action necessary (already a free-standing string).
4. Not owned by old dictionary -> NULL new dictionary
   - No action necessary (already a free-standing string).

* tree.c:
(_copyStringForNewDictIfNeeded): Add.
(xmlSetTreeDoc):
- Update xmlNodePtr->name, xmlNodePtr->content and
  xmlAttrPtr->name when changing the document, if needed.

Found by OSS-Fuzz Issue 45132.