avfilter/vf_ssim360: Use correct type in sizeof

SSIM360Context.ssim360_hist is an array of four pointers to double;
so sizeof(*ssim360_hist[0]) (=sizeof(double)) is the correct size
to use to calculate the amount of memory to allocate, not
sizeof(*ssim360_hist) (which is sizeof(double*)).

Use FF_ALLOCZ_TYPED_ARRAY to avoid this issue altogether.

Fixes Coverity issue #1520671.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_ssim360.c

@@ -1624,7 +1624,7 @@ static int config_output(AVFilterLink *outlink)
         memset(s->ssim360_percentile_sum, 0, sizeof(s->ssim360_percentile_sum));
 
         for (int i = 0; i < s->nb_components; i++) {
-            s->ssim360_hist[i] = av_calloc(SSIM360_HIST_SIZE, sizeof(*s->ssim360_hist));
+            FF_ALLOCZ_TYPED_ARRAY(s->ssim360_hist[i], SSIM360_HIST_SIZE);
             if (!s->ssim360_hist[i])
                 return AVERROR(ENOMEM);
         }