plugins/bcli: plug temporary leak on retry.

Caught by leak detection, we just re-assigned this when we retried: sure,
it's temporary, but it's technically a leak.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/bcli.c

@@ -197,6 +197,7 @@ static void retry_bcli(void *cb_arg)
 	tal_del_destructor(bcli, destroy_bcli);
 
 	list_add_tail(&bitcoind->pending[bcli->prio], &bcli->list);
+	tal_free(bcli->output);
 	next_bcli(bcli->prio);
 }