Follow-up fixes for SHA-2 patch (commit 749a9e20c).

Follow-up fixes for SHA-2 patch (commit 749a9e20c).

This changes the check for valid characters in the salt string to
only allow plain ASCII letters and digits.  The previous coding was
locale-dependent which doesn't really seem like a great idea here;
moreover it could not work correctly in multibyte encodings.

This fixes a careless pointer-use-after-pfree, too.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: Andres Freund <andres@anarazel.de>
Author: Bernd Helmle <mailings@oopsware.de>
Discussion: https://postgr.es/m/6fab35422df6b6b9727fdcc243c5fa1c667dd3b5.camel@oopsware.de