Propagate error in xmlParseElementChildrenContentDeclPriv

Check return value of recursive calls to
xmlParseElementChildrenContentDeclPriv and return immediately in case
of errors. Otherwise, struct xmlElementContent could contain unexpected
null pointers, leading to a null deref when post-validating documents
which aren't well-formed and parsed in recovery mode.

Fixes #243.
4 jobs for master in 7 minutes and 43 seconds (queued for 1 second)