BUG/MINOR: sink: Use the right field in appctx context in release callback

In the release callback, ctx.peers was used instead of ctx.sft. Concretly,
it is not an issue because the appctx context is an union and these both
fields are structures with a unique pointer. But it will be a problem if
that changes.

This patch must be backported as far as 2.2.

src/sink.c

@@ -603,7 +603,7 @@ void __sink_forward_session_deinit(struct sink_forward_target *sft)
 
 static void sink_forward_session_release(struct appctx *appctx)
 {
-	struct sink_forward_target *sft = appctx->ctx.peers.ptr;
+	struct sink_forward_target *sft = appctx->ctx.sft.ptr;
 
 	if (!sft)
 		return;