fix threading in python binding for wait_for_alert

ChangeLog

@@ -69,6 +69,7 @@
 	  incoming connection
 	* added more detailed instrumentation of the disk I/O thread
 
+	* fix python binding for wait_for_alert
 	* only apply privileged port filter to DHT-only peers
 
 0.15.5 release

bindings/python/src/session.cpp

@@ -214,6 +214,7 @@ namespace
 
     alert const* wait_for_alert(session& s, int ms)
     {
+        allow_threading_guard guard;
         return s.wait_for_alert(milliseconds(ms));
     }