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;
 }