pgsql: The shortcut exit that I recently added to ExecInitIndexScan()

Log Message:
-----------
The shortcut exit that I recently added to ExecInitIndexScan() for
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.

Modified Files:
--------------
    pgsql/src/backend/executor:
        nodeIndexscan.c (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.122&r2=1.123)