Invalid memory reference (SIGSEGV) crash

[philn]

Introduced by commit a581863
It's a racy crash, deep in libopentok. The commit looks good at first glance, this could be a bug in opentok, but I'm filing one here for initial triaging.

(gdb) bt                                                                                                 
#0  0x00007ffff69488c2 in otk_session_invoke_msg () at /usr/lib/libopentok.so                            
#1  0x00007ffff6939ab0 in otk_ev_async_callback () at /usr/lib/libopentok.so                             
#2  0x00007ffff69c4eea in uv.async_event () at /usr/lib/libopentok.so                                    
#3  0x00007ffff69c512d in uv.async_io () at /usr/lib/libopentok.so                                       
#4  0x00007ffff69ce253 in uv.io_poll () at /usr/lib/libopentok.so                                        
#5  0x00007ffff69c5588 in uv_run () at /usr/lib/libopentok.so                                            
#6  0x00007ffff693a251 in otk_ev_thread_main () at /usr/lib/libopentok.so                                
#7  0x00007ffff691f7ea in network_thread_main () at /usr/lib/libopentok.so                               
#8  0x00007ffff671dfa3 in start_thread () at /lib/x86_64-linux-gnu/libpthread.so.0                       
#9  0x00007ffff64c44cf in clone () at /lib/x86_64-linux-gnu/libc.so.6