Broker has to wait for its child.

Index: vsftpd-3.0.2/seccompsandbox.c
===================================================================
--- vsftpd-3.0.2.orig/seccompsandbox.c
+++ vsftpd-3.0.2/seccompsandbox.c
@@ -540,6 +540,7 @@ seccomp_sandbox_setup_postlogin_broker()
   seccomp_sandbox_setup_base();
   seccomp_sandbox_setup_data_connections();
   allow_nr_1_arg_match(__NR_sendmsg, 3, 0);
+  allow_nr(__NR_wait4);
   if (tunable_chown_uploads)
   {
     allow_nr(__NR_fstat);