SNMP: Fix OID truncation and sibling lookup (#2244)

SNMP: Fix OID truncation and sibling lookup (#2244)

Previously, SNMP OIDs could be truncated (peer_Inst didn't update
the length after appending a subid), snmpTreeSiblingEntry() misused
len and sometimes returned the wrong sibling or stepped past the
end.

Now we update the OID length while building, select siblings
by matching name[len] (returning the right neighbor only if present).

Also, cleanup socket closure.