- index_merge/intersection is unable to work on GIS indexes, because:
  1. index scans have no Rowid-Ordered-Retrieval property
  2. When one does an index-only read over a GIS index, they do not 
     get the index tuple, because index only contains bounding box of the geometry.
     This is why key_copy() call crashed.
This patch fixes #1, which makes the problem go away. Theoretically, it would 
be nice to check #2, too, but SE API semantics is not sufficiently precise to do it.