avutil/tx: zero whole array, not only one element

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

libavutil/tx.c

@@ -284,7 +284,7 @@ static void reset_ctx(AVTXContext *s, int free_sub)
      * ff_tx_init_subtx() call is made. */
     s->nb_sub = 0;
     s->opaque = NULL;
-    memset(s->fn, 0, sizeof(*s->fn));
+    memset(s->fn, 0, sizeof(s->fn));
 }
 
 void ff_tx_clear_ctx(AVTXContext *s)