libplugin-pay: fix (transitory) memleak which memleak detection compl…

…ains about.

We assign this in the loop without freeing it first.

```
 plugin-pay: MEMLEAK: 0x55792b155e18
 plugin-pay:   label=plugins/libplugin-pay.c:3274:struct short_channel_id_dir
 plugin-pay:   backtrace:
 plugin-pay:     ccan/ccan/tal/tal.c:442 (tal_alloc_)
 plugin-pay:     plugins/libplugin-pay.c:3274 (direct_pay_listpeerchannels)
 plugin-pay:     plugins/libplugin.c:860 (handle_rpc_reply)
 plugin-pay:     plugins/libplugin.c:1036 (rpc_read_response_one)
 plugin-pay:     plugins/libplugin.c:1060 (rpc_conn_read_response)
 plugin-pay:     ccan/ccan/io/io.c:59 (next_plan)
 plugin-pay:     ccan/ccan/io/io.c:407 (do_plan)
 plugin-pay:     ccan/ccan/io/io.c:417 (io_ready)
 plugin-pay:     ccan/ccan/io/poll.c:453 (io_loop)
 plugin-pay:     plugins/libplugin.c:1893 (plugin_main)
 plugin-pay:     plugins/pay.c:1294 (main)
 plugin-pay:     ../sysdeps/nptl/libc_start_call_main.h:58 (__libc_start_call_main)
 plugin-pay:     ../csu/libc-start.c:381 (__libc_start_main_impl)
 plugin-pay:   parents:
 plugin-pay:     plugins/libplugin-pay.c:3308:struct direct_pay_data
 plugin-pay:     plugins/libplugin.c:1775:struct plugin
```

Signed-off-by: Rusty Russell <[email protected]>

plugins/libplugin-pay.c

@@ -3270,6 +3270,7 @@ static struct command_result *direct_pay_listpeerchannels(struct command *cmd,
 		/* Must have either a local alias for zeroconf
 		 * channels or a final scid. */
 		assert(chan->alias[LOCAL] || chan->scid);
+		tal_free(d->chan);
 		d->chan = tal(d, struct short_channel_id_dir);
 		if (chan->scid) {
 			d->chan->scid = *chan->scid;