swr: zero buffers on allocation

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

libswresample/swresample.c

@@ -358,7 +358,7 @@ static int realloc_audio(AudioData *a, int count){
     av_assert0(a->bps);
     av_assert0(a->ch_count);
 
-    a->data= av_malloc(countb*a->ch_count);
+    a->data= av_mallocz(countb*a->ch_count);
     if(!a->data)
         return AVERROR(ENOMEM);
     for(i=0; i<a->ch_count; i++){