snmp_enum: Treat missing timestamp elements as 0

Timestamps don't always have all the elements we expect. This treats
them as zeroes to ensure that we don't raise silly exceptions in that
case.
Let's try to be consistent about Rapid7 Inc.

According to

http://www.sec.gov/Archives/edgar/data/1560327/000156032712000001/0001560327-12-000001.txt

Rapid7 is actually "Rapid7 Inc" not "Rapid7, LLC" any more.

This does not address the few copyright/license statements around
"Metasploit LLC," whatever that is.
Land #3092, Rapid7, Inc. thing
Land #3088, SNMP timestamp elements fix