Fix a memory leak with last

free memory allocated to 'buf'

Submitted by:	Thomas Rix <trix@juniper.net>
Reviewed by:	ed
Approved by:	sjg (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D9850

usr.bin/last/last.c

@@ -243,7 +243,7 @@ wtmp(void)
 	/* Display them in reverse order. */
 	while (amount > 0)
 		doentry(&buf[--amount]);
-
+	free(buf);
 	tm = localtime(&t);
 	(void) strftime(ct, sizeof(ct), "%+", tm);
 	printf("\n%s begins %s\n", ((file == NULL) ? "utx.log" : file), ct);