pgsql: Fix plpgsql so that when a local variable has no initial-value

Log Message:
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Fix plpgsql so that when a local variable has no initial-value expression,
an error will be thrown correctly if the variable is of a NOT NULL domain.
Report and almost-correct fix from Sergiy Vyshnevetskiy (bug #2948).

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.180.2.2 -> r1.180.2.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.180.2.2&r2=1.180.2.3)
    pgsql/src/test/regress/expected:
        domain.out (r1.38 -> r1.38.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/domain.out.diff?r1=1.38&r2=1.38.2.1)
    pgsql/src/test/regress/sql:
        domain.sql (r1.21 -> r1.21.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/domain.sql.diff?r1=1.21&r2=1.21.2.1)