Use Py_XDECREF for pynode_names_and_namespaces (#673)
Use Py_XDECREF for pynode_names_and_namespaces (#673)

As pointed out by clang static analysis, it is possible for
pynode_names_and_namespaces to be NULL during cleanup.  Thus
use Py_XDECREF everywhere to do that checking for us.

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