The attached patch adds a check for the invalid comma and emits a 
runtime error if -std=f95,f2003,f2018 are specified at compile time.

Attached patch includes a new test case.

Regression tested on x86_64-linux-gnu.

OK for mainline?

Regards,

Jerry

Author: Jerry DeLisle <jvdelisle@gcc.gnu.org>
Date:   Fri May 5 20:12:25 2023 -0700

     Fortran: Namelist read with invalid input accepted.

             PR fortran/109662

     libgfortran/ChangeLog:

             * io/list_read.c: Add a check for a comma after a namelist
             name in read input. Issue a runtime error message.

     gcc/testsuite/ChangeLog:

             * gfortran.dg/pr109662.f90: New test.