[J3] CFI_establish and elem_len for zero-length character strings and empty types/structs

[Tobias Burnus]

Hi all,

"" or ["", ""]  has len(...) == 0. For the C array descriptor,
that matches elem_len = 0.

Likewise,
   type, bind(C) :: t; end type
and
   struct t {};
have c_sizeof()/sizeof() == 0, which implies elem_len = 0.

However, CFI_establish requires:
"elem_len – If type is equal to CFI_type_struct, CFI_type_other, or a Fortran
  character type code, elem_len shall be greater than zero and equal to the
  storage size in bytes of an element of the object."

The other CFI_... functions do not have the > 0 requirement (or != 0, as
size_t is unsigned and, hence, implies >= 0).

Tobias