More bounds and length checks.

More bounds and length checks.

Catch INTEGER values with a length of 0, so we don't fetch a byte that
doesn't belong to the value.

Fix what appears to be a long-standing bug in the OID prefix matching
code, wherein the length of the *first* prefix in the table is used as
the length of *all* prefixes, and add some packet-length checking to
that list.

Report packets with an invalid SNMP version number as being SNMP, so
we at least indicate *that*.