pgsql: Fix bugs in sqlchar_to_unicode and unicode_to_sqlchar: both were

Log Message:
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Fix bugs in sqlchar_to_unicode and unicode_to_sqlchar: both were measuring
the length of a UTF8 character with pg_mblen (wrong if DB encoding isn't
UTF8), and the latter was blithely assuming that a static buffer would somehow
revert to all zeroes for each use.

Modified Files:
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    pgsql/src/backend/utils/adt:
        xml.c (r1.80 -> r1.81)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.80&r2=1.81)