scrubbing timeline without jack can crash dummy driver

[ccherrett]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe4f8f700 (LWP 7428)]
0x0000000000762c01 in std::_List_const_iterator<Msg>::operator++() ()
(gdb) bt
#0  0x0000000000762c01 in std::_List_const_iterator<Msg>::operator++() ()
#1  0x0000000000762add in std::iterator_traits<std::_List_const_iterator<Msg> >::difference_type std::__distance<std::_List_const_iterator<Msg> >(std::_List_const_iterator<Msg>, std::_List_const_iterator<Msg>, std::input_iterator_tag) ()
#2  0x00000000007627db in std::iterator_traits<std::_List_const_iterator<Msg> >::difference_type std::distance<std::_List_const_iterator<Msg> >(std::_List_const_iterator<Msg>, std::_List_const_iterator<Msg>) ()
#3  0x0000000000762535 in std::list<Msg, std::allocator<Msg> >::size() const ()
#4  0x0000000000761cf0 in dummyLoop (ptr=0x15e5830) at /home/chris/workspace/git-projects/oom/oom/driver/dummyaudio.cpp:448
#5  0x00007ffff4126a6d in start_thread (arg=<value optimized out>) at pthread_create.c:301
#6  0x00007fffef865d7d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115