Semaphore: check that initial value is not negative

gevent/coros.py

@@ -18,6 +18,8 @@ class Semaphore(object):
     If not given, value defaults to 1."""
 
     def __init__(self, value=1):
+        if value < 0:
+            raise ValueError("semaphore initial value must be >= 0")
         self._links = []
         self.counter = value
         self._notifier = None