pgsql: Fix double-free when XMLSERIALIZE(... INDENT) output exceeds 1GB

Fix double-free when XMLSERIALIZE(... INDENT) output exceeds 1GB.

In xmltotext_with_options(), xmlSaveClose() frees the save context
unconditionally, but ctxt was only reset to NULL on the error branch.
When the close succeeded and a later step threw an error -- typically,
cstring_to_text_with_len() rejecting a result larger than MaxAllocSize
-- control reached the PG_CATCH block with ctxt still pointing at the
freed context, so xmlSaveClose() was called a second time on freed
memory and crashed the backend.

Author: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/bbadb6b2-d226-41a1-ace3-e0eb20648f58@uni-muenster.de
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/582db3ee09e9d2128c2510937bbc3c9f763d97bf

Modified Files
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src/backend/utils/adt/xml.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)