pgcrypto: Detect and report too-short crypt() salts.

Certain short salts crashed the backend or disclosed a few bytes of
backend memory.  For existing salt-induced error conditions, emit a
message saying as much.  Back-patch to 9.0 (all supported versions).

Josh Kupershmidt

Security: CVE-2015-5288

contrib/pgcrypto/sql/crypt-des.sql

@@ -6,6 +6,10 @@ SELECT crypt('', 'NB');
 
 SELECT crypt('foox', 'NB');
 
+-- We are supposed to pass in a 2-character salt.
+-- error since salt is too short:
+SELECT crypt('password', 'a');
+
 CREATE TABLE ctest (data text, res text, salt text);
 INSERT INTO ctest VALUES ('password', '', '');