Fix for ODIDDataSource null object assertion failure processing live open range request (cvp2ri-352)

When processing an open range request for live, a Content Range response
 can't be generated (since the length isn't known - per DLNA 7.5.4.3.2.19.2,
 7.5.4.3.2.20.1). But the code (adapted from the static preroll) was
 assuming a response is always generated.

Tested this fix at PlugFest 37. Issue was no longer observed.