Address FileThreadPool race condition

I saw a couple ThreadErrors around popping from an emtpy queue in
bugsnag: this got introduced when I moved away from catching all
ThreadErrors. There's a race condition between checking that the queue
is empty & attempting to pop from it.

This change locks the pop operation with a mutex to prevent the race.

Author: wfleming

lib/cc/engine/analyzers/file_thread_pool.rb

@@ -14,11 +14,13 @@ def initialize(files, concurrency: DEFAULT_CONCURRENCY)
 
         def run(&block)
           queue = build_queue
+          lock = Mutex.new
 
           @workers = thread_count.times.map do
             Thread.new do
-              while !queue.empty? && (item = queue.pop(true))
-                yield item
+              while !queue.empty?
+                item = lock.synchronize { queue.pop(true) if !queue.empty? }
+                yield item if item
               end
             end
           end