isolate-workers bug

AnyConnect iOS fails to sync .xml profile, if isolate-workers is enabled.

System: Ubuntu 18.04
Kernel: 4.19.4
Ocserv: latest commit c1cb9c02

--10468-- WARNING: unhandled amd64-linux syscall: 317
--10468-- You may be able to write your own handler.
--10468-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--10468-- Nevertheless we consider this a bug.  Please report
--10468-- it at http://valgrind.org/support/bug_reports.html.

valgrind: m_scheduler/sema.c:104 (vgModuleLocal_sema_down): Assertion 'sema->owner_lwpid != lwpid' failed.

host stacktrace:
==10468==    at 0x580441BA: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)

sched status:
  running_tid=0

Thread 1: status = VgTs_WaitSys (lwpid 10468)
==10468==    at 0x65EE48A: prctl (syscall-template.S:78)
==10468==    by 0x51EC313: ??? (in /lib/x86_64-linux-gnu/libseccomp.so.2.3.1)
==10468==    by 0x12ECEF: disable_system_calls (worker-privs.c:123)
==10468==    by 0x11B667: vpn_server (worker-vpn.c:733)
==10468==    by 0x115C99: listen_watcher_cb (main.c:1161)
==10468==    by 0x60B8D72: ev_invoke_pending (in /usr/lib/x86_64-linux-gnu/libev.so.4.0.0)
==10468==    by 0x60BC3DD: ev_run (in /usr/lib/x86_64-linux-gnu/libev.so.4.0.0)
==10468==    by 0x1138D7: main (main.c:1463)