Audio breakups after jitter buffer discarding frames (thanks Bogdan Krakowski for the report)

[nanang]

Audio flood/burst in the beginning of call triggers jitter buffer progressive discard (see ticket #969) to start dropping frames intensively. In the decoding/playing, the dropped frames introduce unaligned signal, so the audio breakups occur.

The proposed solution is by replacing the frames next to the dropped/discarded frames with 'empty' frame to trigger PLC invocation in the decoding so the audio signal will be smoother/aligned by the PLC.

[nanang]

(In [3396]) Fix #1188:

• Fixed jitter buffer progressive discard by returning 'missing' frame after discarded frame(s) so the PLC will be invoked to align the audio signal.
• Modified conditions in jitter buffer test data for this fix.