pgsql: Fix bug in CONTINUE statement for PL/pgSQL: when we continue a

Log Message:
-----------
Fix bug in CONTINUE statement for PL/pgSQL: when we continue a loop,
we need to be careful to reset rc to PLPGSQL_RC_OK, depending on how
the loop's logic is structured. If we continue a loop but it then
exits without executing the loop's body again, we want to return
PLPGSQL_RC_OK to our caller.  Enhance the regression tests to catch
this problem. Per report from Michael Fuhr.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.147 -> r1.148)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.147&r2=1.148)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.35&r2=1.36)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.30&r2=1.31)