avfilter/af_compensationdelay: always initialize w_ptr with 0

It will be changed later anyway, and in case inlink have 0 channels
(should never happen) it will not pick some random value.

libavfilter/af_compensationdelay.c

@@ -127,7 +127,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     const unsigned delay = s->delay;
     const double dry = s->dry;
     const double wet = s->wet;
-    unsigned r_ptr, w_ptr;
+    unsigned r_ptr, w_ptr = 0;
     AVFrame *out;
     int n, ch;