Fix leak of xmlElementContent

* valid.c:
(xmlCopyDocElementContent):
- Set `tmp->parent` properly to fix a leak that occurs in
  xmlFreeDocElementContent().
- Appears to be a regresion from cee2b3a5.

Found by OSS-Fuzz Issue 44509.
6 jobs for ddkilzer/oss-fuzz-44509 in 5 minutes and 36 seconds (queued for 39 seconds)