Guard close() in io callbacks against dead connections

When handle_io_in raises because the connection is already dead
(thisptr == NULL), the error handler called close() which raised a
second ReferenceError. This unhandled exception escaped the cdef
callback, corrupting C-side buffer state and triggering
g_string_erase assertion failures.

Check thisptr before calling close() in both handle_io_in_cb and
handle_io_out_cb error handlers.

Author: micheloosterhof

modules/python/binding.pyx

@@ -825,7 +825,8 @@ cdef int handle_io_in_cb(c_connection *con, void *context, void *data, int size)
 		l = instance.handle_io_in(bdata)
 	except BaseException:
 		logging.error("There was an error in the Python service", exc_info=True)
-		instance.close()
+		if instance.thisptr != NULL:
+			instance.close()
 		return len(bdata)
 	return l
 
@@ -837,7 +838,8 @@ cdef void handle_io_out_cb(c_connection *con, void *context) noexcept with gil:
 		instance.handle_io_out()
 	except BaseException:
 		logging.error("There was an error in the Python service", exc_info=True)
-		instance.close()
+		if instance.thisptr != NULL:
+			instance.close()
 
 cdef c_bool handle_disconnect_cb(c_connection *con, void *context) except * with gil:
 #	print "disconnect_cb"