Guard against race condition on threads keepalive

Requests after the first on a keepalive connection remove themselves
from the keepalive timeout queue. This presents a race condition where
the main thread might try to access the first element of the queue
after it has been removed.

gunicorn/workers/gthread.py

@@ -119,10 +119,11 @@ class ThreadWorker(base.Worker):
     def murder_keepalived(self):
         now = time.time()
         while True:
-            if not len(self._keep):
+            try:
+                delta = self._keep[0].timeout - now
+            except IndexError:
                 break
 
-            delta = self._keep[0].timeout - now
             if delta > 0:
                 break
             else: