avfilter/avf_ahistogram: raise minimum acmax to 1

If acmax can be 0 then it could result in a division by 0
Fixes CID1351345

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avf_ahistogram.c

@@ -165,7 +165,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     const int H = s->histogram_h;
     const int w = s->w;
     int c, y, n, p, bin;
-    uint64_t acmax = 0;
+    uint64_t acmax = 1;
 
     if (!s->out || s->out->width  != outlink->w ||
                    s->out->height != outlink->h) {