segfault in thread-pool.c:246:5:

Building qemu-2.5.0 with -fsanitize=undefined shows, e.g.:

markus@x4 linux % qemu-system-x86_64 -s -enable-kvm -net nic,vlan=0,model=virtio -net user -fsdev local,security_model=none,id=root,path=/ -device virtio-9p-pci,id=root,fsdev
=root,mount_tag=/dev/root -m 512 -smp 2 -kernel /usr/src/linux/arch/x86/boot/bzImage -nographic -append "init=/bin/zsh root=/dev/root console=ttyS0 kgdboc=ttyS0 rootflags=rw,
trans=virtio rootfstype=9p ip=dhcp earlyprintk=ttyS0"
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/exec.c:307:5: runtime error: variable length array bound evaluates to non-positive value 0
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/hw/i386/kvm/apic.c:37:47: runtime error: left shift of 15 by 28 places cannot be represented in type 'int'
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/include/qemu/rcu.h:85:21: runtime error: member access within null pointer of type 'struct rcu_reader_data'
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/include/qemu/rcu.h:101:5: runtime error: member access within null pointer of type 'struct rcu_reader_data'
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/include/qemu/rcu.h:102:8: runtime error: member access within null pointer of type 'struct rcu_reader_data'
...
ALSA device list:
  No soundcards found.
/var/tmp/portage/app-emulation/qemu-2.5.0/work/qemu-2.5.0/thread-pool.c:246:5: runtime error: member access within null pointer of type 'struct ThreadPool'
[1]    9295 segmentation fault  qemu-system-x86_64 -s -enable-kvm -net nic,vlan=0,model=virtio -net user  

As you can see it segfaults when build with upcoming gcc-6, that is more aggressive when it comes to undefined behavior.
The compiler just assumes that "this" can never be NULL and optimizes accordingly.