Fix null OID handling in Agentx on 64bit

The attached patch fixes the handling of null OIDs passed from the subagent to the master.

In agentx_parse_oid() the "0.0" is assigned as to integer (32bit) values "0" to the respective oid objects. As the oid objects are of type long (64bit) only 32bit are assigned and the int_ptr++ only advances by another 32bit instead of 64bit.

The proposed fix uses memset() with 2*sizeof(oid) which should always be correct independent from object sizes.