avcodec/libxavs: fix division by 0 bitrate

Fixes CID1257655
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libxavs.c

@@ -325,6 +325,7 @@ static av_cold int XAVS_init(AVCodecContext *avctx)
     if (avctx->level > 0)
         x4->params.i_level_idc = avctx->level;
 
+    if (avctx->bit_rate > 0)
         x4->params.rc.f_rate_tolerance =
             (float)avctx->bit_rate_tolerance/avctx->bit_rate;