Fortran: error recovery simplifying PACK with invalid arguments [PR106049]

gcc/fortran/ChangeLog:

	PR fortran/106049
	* simplify.cc (is_constant_array_expr): A non-zero-sized constant
	array shall have a non-empty constructor.  When the constructor is
	empty or missing, treat as non-constant.

gcc/testsuite/ChangeLog:

	PR fortran/106049
	* gfortran.dg/pack_simplify_1.f90: New test.