wavpackenc: reset trailer info on block encoding

In some cases, in particular if several blocks are needed because of
the channel layout (e.g. 2.1), the information used to write the
trailing bits terminating the sample data was not reset.

This would cause potential desync on the decoder, although decoded
samples were actually mostly fine.

Fixes ticket #3879.

libavcodec/wavpackenc.c

@@ -2487,6 +2487,9 @@ static int wavpack_encode_block(WavPackEncodeContext *s,
     struct Decorr *dpp;
     PutByteContext pb;
 
+    if (s->flags & WV_MONO_DATA) {
+        CLEAR(s->w);
+    }
     if (!(s->flags & WV_MONO) && s->optimize_mono) {
         int32_t lor = 0, diff = 0;