avoid null-pointer-subtraction error (#78)

Newer compilers generate warnings/errors about null pointer subtractions being an undefined behavior. Use matching non-zero offsets (as suggested by arigo) to avoid the error.

Author: nitzmahone

src/cffi/recompiler.py

@@ -953,7 +953,7 @@ def _struct_ctx(self, tp, cname, approxname, named_ptr=None):
                 if cname is None or fbitsize >= 0:
                     offset = '(size_t)-1'
                 elif named_ptr is not None:
-                    offset = '((char *)&((%s)0)->%s) - (char *)0' % (
+                    offset = '((char *)&((%s)4096)->%s) - (char *)4096' % (
                         named_ptr.name, fldname)
                 else:
                     offset = 'offsetof(%s, %s)' % (tp.get_c_name(''), fldname)