snmp_get_next works only when requesting oid is valid. Otherwise it fails to
find next entry.

Due to the above the snmpwalk works only when the walk is started from
the existing oid. Since snmp walk scans only oids in the current branch,
there was a need to create a special oid .1.3.

$ snmpwalk -On -c public -v 1 192.168.1.20 1.3
.1.3.6.1.2.1.1.5.0 = STRING: "wrc"
.1.3.6.1.2.1.25.1.1.0 = Timeticks: (792773) 2:12:07.73
.1.3.6.1.2.1.25.1.2.0 = Hex-STRING: 07 B2 00 01 05 00 00 00
.1.3.6.1.4.1.96.100.7.5.1.15.1 = Counter32: 445
.1.3.6.1.4.1.96.100.7.5.1.16.1 = INTEGER: 10
.1.3.6.1.4.1.96.100.7.5.1.17.1 = INTEGER: 174810
.1.3.6.1.4.1.96.100.7.5.1.18.1 = INTEGER: 191115
.1.3.6.1.4.1.96.100.7.5.1.19.1 = INTEGER: 157655
Timeout: No Response from 192.168.1.20

NOTE: Timeout message appears becasue there is no responce from node when
wrong packet is sent.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>