lavfi: fix realloc size computation in avfilter_add_format()

Replace sizeof((*avff)->formats)
with    sizeof(*(*avff)->formats)

as the size of the array element is given by the pointed element
rather than by its pointer.

In particular fix computation with the pending patch when
sizeof(int64_t) != sizeof(int64_t *).

libavfilter/formats.c

@@ -99,7 +99,7 @@ int avfilter_add_format(AVFilterFormats **avff, int fmt)
         return AVERROR(ENOMEM);
 
     fmts = av_realloc((*avff)->formats,
-                      sizeof((*avff)->formats) * ((*avff)->format_count+1));
+                      sizeof(*(*avff)->formats) * ((*avff)->format_count+1));
     if (!fmts)
         return AVERROR(ENOMEM);