pgsql: Free SQLSTATE and SQLERRM no earlier than other PL/pgSQL variabl

Free SQLSTATE and SQLERRM no earlier than other PL/pgSQL variables.

"RETURN SQLERRM" prompted plpgsql_exec_function() to read from freed
memory.  Back-patch to 9.0 (all supported versions).  Little code ran
between the premature free and the read, so non-assert builds are
unlikely to witness user-visible consequences.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/40a9a16760ab803694583d7f3549a7ad7bc11d75

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c          |   12 +++---------
src/test/regress/expected/plpgsql.out |   12 ++++++++++++
src/test/regress/sql/plpgsql.sql      |   10 +++++++++-
3 files changed, 24 insertions(+), 10 deletions(-)