avio: fix sizeof argument

CC: libav-stable@libav.org
Bug-Id: CID 732284

libavformat/avio.c

@@ -361,7 +361,7 @@ int ffurl_get_multi_file_handle(URLContext *h, int **handles, int *numhandles)
     if (!h->prot->url_get_multi_file_handle) {
         if (!h->prot->url_get_file_handle)
             return AVERROR(ENOSYS);
-        *handles = av_malloc(sizeof(*handles));
+        *handles = av_malloc(sizeof(**handles));
         if (!*handles)
             return AVERROR(ENOMEM);
         *numhandles = 1;