codec/rfx: fix multithreaded encoder

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.
Merge pull request #1413 from nfedera/fix-2013-08-10-01

codec/rfx: fix multithreaded encoder