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