wmalosslessdec: reset frame->nb_samples on packet loss

Otherwise a frame with non-zero nb_samples but without any data can be
returned.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 42e7a5b3)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wmalosslessdec.c

@@ -1028,6 +1028,7 @@ static int decode_frame(WmallDecodeCtx *s)
     if ((ret = ff_get_buffer(s->avctx, s->frame, 0)) < 0) {
         /* return an error if no frame could be decoded at all */
         s->packet_loss = 1;
+        s->frame->nb_samples = 0;
         return ret;
     }
     for (i = 0; i < s->num_channels; i++) {