pgsql: Fix crash with old libxml2

Fix crash with old libxml2

Certain libxml2 versions (such as the 2.7.6 commonly seen in older
distributions, but apparently only on x86_64) contain a bug that causes
xmlCopyNode, when called on a XML_DOCUMENT_NODE, to return a node that
xmlFreeNode crashes on.  Arrange to call xmlFreeDoc instead of
xmlFreeNode for those nodes.

Per buildfarm members lapwing and grison.

Author: Pavel Stehule, light editing by Álvaro.
Discussion: https://postgr.es/m/20190308024436.GA2374@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e616dee9e601d36462dc4cc48eb0b6a1ff20051

Modified Files
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src/backend/utils/adt/xml.c | 58 +++++++++++++++++++++++++++++++--------------
1 file changed, 40 insertions(+), 18 deletions(-)