Added parser method used to obtain a 'null' object reference.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5830 acf43c95-373e-0410-b603-e72c3f656dc1
Return null object from XML constructor if parse fails.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5831 acf43c95-373e-0410-b603-e72c3f656dc1
Merge branch 'upstream' of github.com:vir/yate into deb
Updated debian package version to 5.3.1~svn5831