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@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/0fde377a-3870-4d18-946a-ce008ee5bb88@gmail.com