Don't crash if cursor_to_xmlschema is used on a non-data-returning Portal.

Don't crash if cursor_to_xmlschema is used on a non-data-returning Portal.

cursor_to_xmlschema() assumed that any Portal must have a tupDesc,
which is not so.  Add a defensive check.

It's plausible that this mistake occurred because of the rather
poorly chosen name of the lookup function SPI_cursor_find(),
which in such cases is returning something that isn't very much
like a cursor.  Add some documentation to try to forestall future
errors of the same ilk.

Report and patch by Boyu Yang (docs changes by me).  Back-patch
to all supported branches.

Discussion: http://postgr.es/m/dd343010-c637-434c-a8cb-418f53bda3b8.yangboyu.yby@alibaba-inc.com