[CN] crash if array size expression evaluates to a negative number

[peterohanley]

failing cases (array size evaluates to -1)

struct a {
  int b[sizeof(char) - 2];
};

cn: internal error, uncaught exception:
    Z.Overflow
    Raised by primitive operation at Z.to_int in file "z.ml", line 221, characters 46-56
    Called from Cn__Core_to_mucore.make_struct_decl in file "backend/cn/lib/core_to_mucore.ml", line 1457, characters 23-56
    Called from Cn__Core_to_mucore.normalise_tag_definition in file "backend/cn/lib/core_to_mucore.ml", line 1493, characters 53-86
    Called from Cn__Core_to_mucore.normalise_tag_definitions.(fun) in file "backend/cn/lib/core_to_mucore.ml", line 1501, characters 20-52
    Called from Pmap.fold in file "pmap.ml", line 156, characters 15-38
    Called from Cn__Core_to_mucore.normalise_file in file "backend/cn/lib/core_to_mucore.ml", line 1526, characters 17-58
    ...

int b[sizeof(char) - 2];

cn: internal error, uncaught exception:
    Failure("int_of_big_int")
    Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
    Called from Cerb_frontend__Cabs_to_ail_aux.mk_zeroInit_aux in file "ocaml_frontend/generated/cabs_to_ail_aux.ml", line 128, characters 34-56
    Called from Cerb_frontend__Cabs_to_ail_aux.mk_zeroInit in file "ocaml_frontend/generated/cabs_to_ail_aux.ml", line 161, characters 17-47
    Called from Cerb_frontend__Cabs_to_ail_effect.extract_program.(fun) in file "ocaml_frontend/generated/cabs_to_ail_effect.ml", line 2702, characters 51-86
    Called from Cerb_frontend__State_exception.stExpect_bind in file "ocaml_frontend/generated/state_exception.ml", line 19, characters 24-29
    ....

normal cases (array size evaluates to 0)

struct a {
  int b[sizeof(char) - 1];
};

pre.c:2:7: error: constraint violation: array declared with a negative or zero size
  int b[sizeof(char) - 1];
      ^~~~~~~~~~~~~~~~~~~~ 

The error is the same if the array is not inside a struct.