Fix memory leak in QDomDocument DTD entity declaration handler

Fix memory leak in QDomDocument DTD entity declaration handler

The created entity node's reference count needs to be decremented to 0
before it is added as a child, because appendChild will increment the
reference count to correct value of 1. Also added autotest DTDEntityDecl
to tst_qdom to expose the leak when executed under valgrind memcheck.
There was no previous direct test case for unparsed entity declarations in
DTD, only indirect coverage via regression test cloneDTD_QTBUG8398.

Task-number: QTBUG-22587
Change-Id: I394ae9fc32d5b84e4ca287c5db4dd7effde6128b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>