Default to current window in jump_to_item

[elkowar]

This PR fixes an error where sometimes, jump_to_item would crash in case the window-id was nil for some reason.
By just defaulting to 0 in that case, we can avoid that error trivially.
This error occurred when auto_jump was used.
The fix does not introduce any other breaking changes or behavior changes.

[hehelego]

Seems this plugin is no longger maintained and the creator haven't been active on github for months.
Perhaps we should start a maintained fork?

[kluen]

I'm wondering if it would make more sense to find the places where we get nil values and fix them. On the one hand this change would make the errors disappear but also mask the true reasons, which could also be bugs.