ExodusII_IO::set_output_variables may pass in invalid variable names

[YaqiWang]

Currently the function directly sets a member variable. It may create unexpected problems when this variables names are used. My suggestion is that whenever the names are used, we check the validity of the names with the passed in equation system. One issue was reported in MOOSE https://github.com/idaholab/moose/issues/20537.