Don't spawn too many consumers

If len(to_process) < num_processes, some would not be able to get an
item from the queue, thereby blocking indefinitely. In that case, use
only as many consumers as necessary to process all the files.

abzer/abzer.py

@@ -137,7 +137,8 @@ async def run(self):
         if to_process:
             tasks = []
             tasks.append(self.producer(to_process))
-            for i in range(0, self.num_processes):
+            num_tasks = min(len(to_process), self.num_processes)
+            for i in range(0, num_tasks):
                 tasks.append(self.consumer())
             tasks.append(self.queue.join())
             await asyncio.gather(*tasks)