Fix segfault that occuued when seaching for a common ancestor when the second window (i.e. the new window with mouse) is 0.  See: http://www.cegui.org.uk/phpBB2/viewtopic.php?f=3&t=5079
Fix segfault that occuued when seaching for a common ancestor when the second window (i.e. the new window with mouse) is 0.  See: http://www.cegui.org.uk/phpBB2/viewtopic.php?f=3&t=5079

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branch : v0-7

cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two