revid-cli: zero-length MTS data sent

[Alan Noble]

Occasionally revid-cli sends /recv requests with zero-length data, triggering the following log messages in VidGrind.

2019/08/15 15:25:44 /recv?vn=142&ma=00:E0:4C:00:00:10&dk=25665160&ut=3639&V0=0
2019/08/15 15:25:44 writeMtsMedia(15413563883776) PSI not found, len=0

[Saxon Milton]

This was an issue when writing frames too big for ring buffer, but now with variable ring buffer, this doesn't happen.