Pop from an empty deque, 100 processes option

### Description

I'm getting this error, the execution stops and nothing else.

```
Traceback (most recent call last):
File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 720, in next
item = self._items.popleft()
IndexError: pop from an empty deque
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 726, in next
item = self._items.popleft()
IndexError: pop from an empty deque
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/app/scancode-toolkit/bin/scancode", line 11, in
load_entry_point('scancode-toolkit', 'console_scripts', 'scancode')()
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File "/app/scancode-toolkit/src/scancode/utils.py", line 70, in main
standalone_mode=standalone_mode, **extra)
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File "/app/scancode-toolkit/src/scancode/cli.py", line 519, in scancode
*args, **kwargs)
File "/app/scancode-toolkit/src/scancode/cli.py", line 914, in run_scan
quiet=quiet, verbose=verbose, kwargs=requested_options, echo_func=echo_func,
File "/app/scancode-toolkit/src/scancode/cli.py", line 1105, in run_scanners
with_timing=timing, progress_manager=progress_manager)
File "/app/scancode-toolkit/src/scancode/cli.py", line 1199, in scan_codebase
location, rid, scan_errors, scan_time, scan_result, scan_timings = next(scans)
File "/app/scancode-toolkit/lib/python3.6/site-packages/click/_termui_impl.py", line 259, in next
rv = next(self.iter)
File "/app/scancode-toolkit/src/scancode/pool.py", line 48, in wrap
return func(self, timeout=timeout or 3600)
File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 730, in next
raise TimeoutError
multiprocessing.context.TimeoutError
/app/scancode-toolkit/scancode -clpi -n 100 --json-pp /app/output.json /app/workdir: b''
Error: None
````

### How To Reproduce

```
scancode -clpi -n 100 --json-pp /app/output.json /app/source_code
```

### System configuration

* Running on a python:3.6 docker image on an ec2 instance with 16 CPUs 150 GB SSD y 200 EBS volume, 16GB of ram.
* We build from the source from the develop branch on every run.