Make extract_stack resilient to lacking frames. (#563)

uvloop/cbhandles.pyx

@@ -414,7 +414,12 @@ cdef extract_stack():
     """Replacement for traceback.extract_stack() that only does the
     necessary work for asyncio debug mode.
     """
-    f = sys_getframe()
+    try:
+        f = sys_getframe()
+    # sys._getframe() might raise ValueError if being called without a frame, e.g.
+    # from Cython or similar C extensions.
+    except ValueError:
+        return None
     if f is None:
         return