avcodec/zmbv: Update decomp_len in raw frames

decomp_len is used in raw frames, so it should not be left at the value from
whatever was decoded previously (which may be any other frame)

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3d201b83cd)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/zmbv.c

@@ -519,6 +519,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
             return AVERROR_INVALIDDATA;
         }
         memcpy(c->decomp_buf, buf, len);
+        c->decomp_len = len;
     } else { // ZLIB-compressed data
         c->zstream.total_in = c->zstream.total_out = 0;
         c->zstream.next_in = (uint8_t*)buf;