Use Py_XDECREF for pytopic_names_and_types. (#638)
Use Py_XDECREF for pytopic_names_and_types. (#638)

If we got here through PyList_New failing, then pytopic_names_and_types
could be NULL.  Therefore, we need to use Py_XDECREF to handle
the situation.  Pointed out by clang static analysis.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@openrobotics.org>
ros2/rclpy,ros2/rclpy