[RDY] coverity/62620: fix uninitialized scalar variable

This PR fixes a coverity issue identified by @aktau in #717.
The Position variable is now initialized the same way in both window_get_position() and window_get_cursor().
Hopefully a straightforward issue, but comments welcome.
We actually have `POSITION_INIT` for this purpose::

    Position rv = POSITION_INIT;

Could you change to this?