ev-window-title: Avoid a crash caused by double free

Variable "title_header" of ev_window_title_update() should not be freed at the end of the function as it points to window_title->doc_title.

This crash happens when closing Evince (with different backtraces). There is quite a lot of occurences of the crashes in our bugzilla, see https://bugzilla.redhat.com/show_bug.cgi?id=2178016 and its duplicates.

Fixes #1912 (closed)