snmpd, fix segfault when walking off the end of OPENBSD-MEM-MIB::memIfName

[Stuart Henderson]

$ snmpwalk -v2c -c public $SOMEHOST OPENBSD-MEM-MIB::memIfName
<segfault>

introduced when per-interface livelock counters were removed.
the variable should probably be removed completely sometime, but is
this ok for now?

Index: mib.c
===================================================================
RCS file: /cvs/src/usr.sbin/snmpd/mib.c,v
retrieving revision 1.42
diff -u -p -r1.42 mib.c
--- mib.c       23 Sep 2010 10:49:55 -0000      1.42
+++ mib.c       15 Oct 2010 09:39:39 -0000
@@ -1428,7 +1428,8 @@ mib_memiftable(struct oid *oid, struct b
                ber = ber_add_string(ber, kif->if_name);
                break;
        case 2:
-               ber = ber_add_string(ber, 0);
+               ber = ber_add_integer(ber, 0);
+               ber_set_header(ber, BER_CLASS_APPLICATION, SNMP_T_COUNTER64);
                break;
        default:
                return (-1);