pgsql: Don't crash if cursor_to_xmlschema is used on a non-data-returni

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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/95f54f0d034eb69349f9134a01441a3cdd311419

Modified Files
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doc/src/sgml/spi.sgml | 13 +++++++++++++
src/backend/utils/adt/xml.c | 4 ++++
2 files changed, 17 insertions(+)