gdb/python: Avoid use after free in py-tui.c

gdb/python: Avoid use after free in py-tui.c

When setting the window title of a tui frame we do this:

  gdb::unique_xmalloc_ptr<char> value
    = python_string_to_host_string (<python-object>);
  ...
  win->window->title = value.get ();

The problem here is that 'get ()' only borrows the pointer from value,
when value goes out of scope the pointer will be freed.  As a result,
the tui frame will be left with a pointer to undefined memory
contents.

Instead we should be using 'value.release ()' to take ownership of the
pointer from value.

gdb/ChangeLog:

	* python/py-tui.c (gdbpy_tui_set_title): Use release, not get, to
	avoid use after free.