xmlNodeSetName: Allow setting the name to a substring of the currently set name

Avoid freeing the currently set name until after having assigned the new name,
this allows one to call xmlNodeSetName (node, node->name + 1) to set the new
name of the node to a substring of the current name without introducing any
crash and without requiring an extra strdup().
Merge branch 'master' of git://git.gnome.org/libxml2 into os400