abort in circuitmux_detach_all_circuits on Tor Browser exit with stem scripts running
Using master (0.2.8.0-alpha-dev) on OS X I get a crash in circuitmux_detach_all_circuits when I exit Tor Browser (modified to use the binary from a master build) while running a script similar to stem's custom_path_selection.py example.
```
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff8d787002 __pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fff96ce75c5 pthread_kill + 90
2 libsystem_c.dylib 0x00007fff9de5f6e7 abort + 129
3 tor 0x00000001036b3e57 circuitmux_detach_all_circuits + 743
4 tor 0x00000001036a0abc channel_free_list + 236
5 tor 0x00000001036a0863 channel_free_all + 147
6 tor 0x0000000103727a43 tor_free_all + 131
7 tor 0x0000000103727cb8 tor_cleanup + 296
8 tor 0x0000000103727262 signal_callback + 2498
9 libevent-2.0.5.dylib 0x0000000103902be0 event_signal_closure + 185
10 libevent-2.0.5.dylib 0x00000001039029f7 event_process_active_single_queue + 237
11 libevent-2.0.5.dylib 0x00000001039030b1 event_process_active + 122
12 libevent-2.0.5.dylib 0x00000001039036ca event_base_loop + 647
13 tor 0x0000000103725aac do_main_loop + 1532
14 tor 0x0000000103727dbf tor_main + 223
15 tor 0x00000001036943a9 main + 25
16 libdyld.dylib 0x00007fff903cb5ad start + 1
```
issue