indeo3: error out if no motion vector is set.

This fixes a crash on a corrupt bitstream (bugzilla #93).
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/indeo3.c

@@ -756,6 +756,8 @@ static int parse_bintree(Indeo3DecodeContext *ctx, AVCodecContext *avctx,
                     av_log(avctx, AV_LOG_ERROR, "SkipCell procedure not implemented yet!\n");
 
                 CHECK_CELL
+                if(!curr_cell.mv_ptr)
+                    return AVERROR_INVALIDDATA;
                 copy_cell(ctx, plane, &curr_cell);
                 return 0;
             }