pgsql: Minor cleanup/future-proofing for pg_saslprep().

Minor cleanup/future-proofing for pg_saslprep().

Ensure that pg_saslprep() initializes its output argument to NULL in
all failure paths, and then remove the redundant initialization that
some (not all) of its callers did. This does not fix any live bug,
but it reduces the odds of future bugs of omission.

Also add a comment about why the existing failure-path coding is
adequate.

Back-patch so as to keep the function's API consistent across branches,
again to forestall future bug introduction.

Patch by me, reviewed by Michael Paquier

Discussion: https://postgr.es/m/16558.1536407783@sss.pgh.pa.us

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/86d226ae5a3607b8bbde2487f821b8f8807da1c8

Modified Files
--------------
src/backend/libpq/auth-scram.c | 4 ++--
src/common/saslprep.c | 11 ++++++++---
src/interfaces/libpq/fe-auth-scram.c | 4 ++--
3 files changed, 12 insertions(+), 7 deletions(-)