indeo4: check band->scan

Fixes null ptr exception

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/indeo4.c

@@ -444,6 +444,11 @@ static int decode_band_hdr(IVI4DecContext *ctx, IVIBandDesc *band,
 
     align_get_bits(&ctx->gb);
 
+    if (!band->scan) {
+        av_log(avctx, AV_LOG_ERROR, "band->scan not set\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     return 0;
 }