Fix ld-decode deadlock on exit.

On machines with small numbers of CPUs, ld-decode would sometimes
deadlock on exit, with two threads in different processes blocked
reading from pipes.

This turned out to be a race in the DemodCache destructor. It was doing:

for i in self.threads:
    if i.is_alive():
        self.q_in.put(None)

with the workers exiting when they received a None message. However,
suppose you've got two workers, and the following happens:

- DemodCache checks if A is alive - it is
- DemodCache sends None
- B receives None and exits
- DemodCache checks if B is alive - it isn't

A is now stuck waiting for a message that'll never arrive. I've fixed
this by removing the is_alive checks -- so it may now send more None
messages than necessary, but that's harmless.

lddecode/core.py

@@ -752,12 +752,10 @@ def __init__(self, rf, infile, loader, cachesize = 128, num_worker_threads=6, MT
     def end(self):            
         # stop workers
         for i in self.threads:
-            if i.is_alive():
-                self.q_in.put(None)
+            self.q_in.put(None)
 
         for t in self.threads:
-            if t.is_alive():
-                t.join()
+            t.join()
 
         self.q_out.put(None)