Some component of the encoder chain (I suspect the rlgr encoder) expects
the output buffer to be zeroed. The multithreaded RemoteFX encoder uses
wStreams from the StreamPool which are reused and not zeroed out of
course. For now, in order to prevent data corruption we clear the stream.
(cherry picked from commit ccc5d1b279eb76721adde44576f8226c418abd23)
df7311b4f9