pgsql: Fix oversight in commit 0dca5d68d.

Fix oversight in commit 0dca5d68d.

As coded, fmgr_sql() would get an assertion failure for a SQL function
that has an empty body and is declared to return some type other than
VOID. Typically you'd never get that far because fmgr_sql_validator()
would reject such a definition (I suspect that's how come I managed to
miss the bug). But if check_function_bodies is off or the function is
polymorphic, the validation check wouldn't get made.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/0fde377a-3870-4d18-946a-ce008ee5bb88@gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/dbd437e670b88ca67f6b2477e853412a0e82a8cc

Modified Files
--------------
src/backend/executor/functions.c | 15 ++++++++++++++-
src/test/regress/expected/create_function_sql.out | 17 ++++++++++++++++-
src/test/regress/sql/create_function_sql.sql | 10 ++++++++++
3 files changed, 40 insertions(+), 2 deletions(-)