allow passing a null through a cstr arg/return

tests-pl/ffi_cstr.pl

@@ -1,16 +1,21 @@
 :- use_module(library(os)).
 :- use_module(library(ffi)).
 
-test :- 
+init :-
     read(Body),
     term_variables(Body, [LIB]),
     Body,
     use_foreign_module(LIB, [
         'ffi_cstr_len'([cstr], u64),
-        'ffi_example_cstr'([], cstr)
-    ]),
+        'ffi_example_cstr'([], cstr),
+        'ffi_null_cstr'([], cstr)
+    ]).
+
+test :-
     ffi:'ffi_cstr_len'("Scryer Prolog", Len),
     ffi:'ffi_example_cstr'(Str),
-    write((Len-Str)).
+    ffi:'ffi_null_cstr'(Null),
+    ffi:'ffi_cstr_len'(0, MaxU64),
+    write((Len-Str-Null-MaxU64)).
 
-:- initialization(test).
+:- initialization((init,test)).