check if ignore_unset_args is set in xmlloader

[knorth55]

ignore_unset_args is introduced in #1788, but it is set from outside of the class instance. https://github.com/ros/ros_comm/commit/3fc9b9ba909f9c691897d41401c3eaf47ad6ef02#diff-3157de68f88b12267c21f7208ae4c58c5c9bd4ab1ca2685c2167bc46e96daeb8R445

This PR change to check ignore_unset_args attribute is set before calling it.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/knorth55/.cache/dein/.cache/.vimrc/.dein/plugin/rosvim/filetypes/launch.py", line 150, in syntastic_checker
    loader.load(vimp.buf.name, conf, verbose=False)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 763, in load
    if not self.ignore_unset_args:
AttributeError: 'XmlLoader' object has no attribute 'ignore_unset_args'

my environment: ROS: melodic Ubuntu 18.04 roslaunch: 1.14.12-1bionic.20210921.202835