swresample: fix negative rematrix volumns

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

libswresample/rematrix.c

@@ -313,7 +313,7 @@ av_cold static int auto_matrix(SwrContext *s)
     } else
         maxval = INT_MAX;
 
-    if(maxcoef > maxval){
+    if(maxcoef > maxval || s->rematrix_volume  < 0){
         maxcoef /= maxval;
         for(i=0; i<SWR_CH_MAX; i++)
             for(j=0; j<SWR_CH_MAX; j++){