[rbp/omxplayer] Fix uninitialised variable that can lose audio packet

The uninitialised variable meant the first run through Decode goes through the non-passthrough path (when passthrough)
and so doesn't get correctly submitted and is discarded
Merge pull request #3071 from popcornmix/uninit_audio

[rbp/omxplayer] Fix uninitialised variable that can lose audio packet