#1396 Correct the value of registered OID

[Gao Yahu]

Replaced 'name_a' and 'namelen' with 'start_a' and 'start_len'.
'name_a' and 'namelen' are used to define the objid prefix of registered subtree. Some nodes in the registered tree will be skipped while reattaching nodes with 'name_a' and 'namelen'.
For example iso.3.6.1.2.1.4.21.1.14, its objid prefix is 'IP-MIB::ip' and objid is 'IP-MIB::ip.21.1.14'.