indeo5: prevent null pointer dereference on broken files

Found by John Villamil <johnv@matasano.com>

libavcodec/indeo5.c

@@ -453,6 +453,10 @@ static int decode_mb_info(IVI5DecContext *ctx, IVIBandDesc *band,
     ref_mb = tile->ref_mbs;
     offs   = tile->ypos * band->pitch + tile->xpos;
 
+    if (!ref_mb &&
+        ((band->qdelta_present && band->inherit_qdelta) || band->inherit_mv))
+        return AVERROR_INVALIDDATA;
+
     /* scale factor for motion vectors */
     mv_scale = (ctx->planes[0].bands[0].mb_size >> 3) - (band->mb_size >> 3);
     mv_x = mv_y = 0;