avcodec/cinepak: Add missing const

Found-by: <mkver>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cinepak.c

@@ -354,7 +354,7 @@ static int cinepak_predecode_check (CinepakContext *s)
         return AVERROR_INVALIDDATA;
 
     if (num_strips) {
-        uint8_t *data = s->data + 10 + s->sega_film_skip_bytes;
+        const uint8_t *data = s->data + 10 + s->sega_film_skip_bytes;
         int strip_size = AV_RB24 (data + 1);
         if (strip_size < 12 || strip_size > encoded_buf_size)
             return AVERROR_INVALIDDATA;