Make sure necessary cleanups such as uv_mutex_unlock are called always in GetSerialDevices().

[enami]

In the function GetSerialDevices() (in darwin_list.cpp), there are code paths that rest of iteration is skipped by continue statements. But by taking such paths, some necessary cleanups are also skipped (e.g., uv_mutex_unlock). And missing uv_mutex_unlock(), for example, results dead locks on next iteration.

This change makes sure that

1. IOObjectRelease(device) is always called if device was not NULL,
2. uv_mutex_unlock(&list_mutex) is always called if uv_mutex_lock(&list_mutex) was called, and
3. IOObjectRelease(modemService) is always called if modemService was not NULL.

[reconbot]

seems to work for me 👍