Work around bootstrap failure in Fortran front end.

Work around bootstrap failure in Fortran front end.

Switching the C++ front end to lower loops the same was as the C front
end triggered this error when bootstrapping the Fortran front end:

/path/to/gcc/fortran/interface.c:3546:12: error: '*new_arg' may be used uninitialized [-Werror=maybe-uninitialized]
 3546 |   new_arg[i]->next = NULL;
      |   ~~~~~~~~~^

Work around this by adding an assertion, which seems appropriate for
documentation and good coding practices anyway.

2020-08-12  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/fortran/
	* interface.c (gfc_compare_actual_formal): Add assertion after
	main processing loop to silence maybe-uninitialized error.