Fix NoMethodError if handler has no ParentModule

db.rb assumes that multi/handler sessions have a ParentModule defined
in their datastore. This assumption breaks when a user sets up a
multi/handler by hand to receive a session from another user (e.g.,
via multi_meter_inject).

When db.rb tries to access a member of a nil ParentModule, a
stacktrace is dumped to framework.log.
Merge branch 'fixmh' of github.com:rsmudge/metasploit-framework into rsmudge-fixmh