Make DISCARD SEQUENCES also discard the last used sequence.

Otherwise, we access already-freed memory.  Oops.

Report by Michael Paquier.  Fix by me.

src/backend/commands/sequence.c

@@ -1617,4 +1617,6 @@ ResetSequenceCaches(void)
 		free(seqtab);
 		seqtab = next;
 	}
+
+	last_used_seq = NULL;
 }

src/test/regress/expected/sequence.out

@@ -344,6 +344,9 @@ SELECT lastval();
       99
 (1 row)
 
+DISCARD SEQUENCES;
+SELECT lastval();
+ERROR:  lastval is not yet defined in this session
 CREATE SEQUENCE seq2;
 SELECT nextval('seq2');
  nextval 

src/test/regress/sql/sequence.sql

@@ -154,6 +154,8 @@ SELECT nextval('seq');
 SELECT lastval();
 SELECT setval('seq', 99);
 SELECT lastval();
+DISCARD SEQUENCES;
+SELECT lastval();
 
 CREATE SEQUENCE seq2;
 SELECT nextval('seq2');