pgsql: Don't try to run clauseless index scans on index types that don't

Log Message:
-----------
Don't try to run clauseless index scans on index types that don't support
it. Per report from Marinos Yannikos.

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
indxpath.c (r1.173 -> r1.174)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.173&r2=1.174)