EXPLAIN-only operation was a little too short; it skipped initializing the
node's result tuple type, which may be needed depending on what's above the
indexscan node.  Call ExecAssignResultTypeFromTL before exiting.  (For good
luck I moved up the ExecAssignScanProjectionInfo call as well, so that
everything except indexscan-specific initialization will still be done.)
Per example from Grant Finnemore.