pgsql: Some fine-tuning of xmlpi in corner cases: - correct error codes

Log Message:
-----------
Some fine-tuning of xmlpi in corner cases:
- correct error codes
- do syntax checks in correct order
- strip leading spaces of argument

Modified Files:
--------------
    pgsql/src/backend/executor:
        execQual.c (r1.203 -> r1.204)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.203&r2=1.204)
    pgsql/src/backend/utils/adt:
        xml.c (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.12&r2=1.13)
    pgsql/src/include/utils:
        xml.h (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h.diff?r1=1.6&r2=1.7)
    pgsql/src/test/regress/expected:
        xml.out (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.3&r2=1.4)
        xml_1.out (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.4&r2=1.5)
    pgsql/src/test/regress/sql:
        xml.sql (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.3&r2=1.4)