Previously, the pts value was initialised to AV_NOPTS_VALUE and so
it was not necessary to always set it. Now, with the new-new decode
API, this is no longer true. I'm not sure why I avoided setting the
pts when the decoder value was also AV_NOPTS_VALUE - it clearly
wouldn't have changed anything previously, but here we are.

Failing to do this, means the frame pts will be some random uninitalised
value.