vc1dec: Use the right pointer type for the tmp pointer

This fixes warnings about assignment from incompatible
pointer type.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -5426,13 +5426,12 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
     AVFrame *pict = data;
     uint8_t *buf2 = NULL;
     const uint8_t *buf_start = buf;
-    uint8_t *tmp;
     int mb_height, n_slices1;
     struct {
         uint8_t *buf;
         GetBitContext gb;
         int mby_start;
-    } *slices = NULL;
+    } *slices = NULL, *tmp;
 
     /* no supplementary picture */
     if (buf_size == 0 || (buf_size == 4 && AV_RB32(buf) == VC1_CODE_ENDOFSEQ)) {