pgsql: Fix EXIT out of outermost block in plpgsql.

Fix EXIT out of outermost block in plpgsql.

Ordinarily, using EXIT this way would draw "control reached end of
function without RETURN". However, if the function is one where we
don't require an explicit RETURN (such as a DO block), that should
not happen. It did anyway, because add_dummy_return() neglected to
account for the case.

Per report from Herwig Goemans. Back-patch to all supported branches.

Discussion: https://postgr.es/m/868ae948-e3ca-c7ec-95a6-83cfc08ef750@gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b1de90699e73a081ac450420b7d4f3fd1d49981b

Modified Files
--------------
src/pl/plpgsql/src/expected/plpgsql_control.out | 11 +++++++++++
src/pl/plpgsql/src/pl_comp.c | 6 ++++--
src/pl/plpgsql/src/sql/plpgsql_control.sql | 12 ++++++++++++
3 files changed, 27 insertions(+), 2 deletions(-)