[O] [BUG][ODT] Subtree export fails when link references target on higher level

[Christian Moe]

The two examples below illustrate an odd ODT export failure that occurs in
restricted circumstances: only when the target of the link is on a
higher level of the tree than the link. 

* This fails

** A subheader

This paragraph is bookmarked. <<target1>> 

When I try to export this subtree to ODT (`C-c C-e C-s o O' at point), I get
: user-error: No such file: /Users/CM/org/OpenDocument export failed: FIXME?

*** A sub-subheader

Here's a ref to the bookmark at [[target1]].

* This works

Here, I reverse the positions of reference and target from the first example.

** A subheader

Here's a ref to the bookmark at [[target2]].

I can do subtree export from here with no problem.

*** A sub-subheader

This paragraph is bookmarked. <<target2>>