Remove the special casing of Py_None when converting the return value
of the Python part of a callback function to C.  If it cannot be
converted, call PyErr_WriteUnraisable with the exception we got.
Before, arbitrary data has been passed to the calling C code in this
case.

(I'm not really sure the NEWS entry is understandable, but I cannot
find better words)
diff --git a/Misc/NEWS b/Misc/NEWS
index ab40d3d..dc99ff7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -64,6 +64,11 @@
 Library
 -------
 
+- If a the Python part of a ctypes callback function returns None,
+  and this cannot be converted to the required C type, an exception is
+  printed with PyErr_WriteUnraisable.  Before this change, the C
+  callback did return arbitrary values to the calling code.
+
 - The __repr__ method of a NULL ctypes.py_object() no longer raises
   an exception.