pgsql: Some code review for xml.c: Add some more xml_init() calls that

Log Message:
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Some code review for xml.c:
Add some more xml_init() calls that might not be necessary, but seem like a
good idea to avoid possible problems like we saw in xmlelement().
Fix unsafe assumption that you can keep using the tupledesc of a relcache
entry you don't have open.
Add missing error checks for SearchSysCache failure.
Get rid of handwritten array traversal in xpath() and O(N^2), broken-for-nulls
array access code in map_sql_value_to_xml_value(), in favor of using
deconstruct_array.
Manually adjust a lot of line breaks in places where the code is otherwise
gonna look pretty awful after pg_indent hacks it up (original author seems to
have liked to lay out code for a 200-column window).

Modified Files:
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    pgsql/src/backend/utils/adt:
        xml.c (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.50&r2=1.51)