[3.6] bpo-13617: Reject embedded null characters in wchar* strings. (GH-2302) (#2462)

Based on patch by Victor Stinner.

Add private C API function _PyUnicode_AsUnicode() which is similar to
PyUnicode_AsUnicode(), but checks for null characters..
(cherry picked from commit f7eae0adfcd4c50034281b2c69f461b43b68db84)
diff --git a/Lib/test/test_time.py b/Lib/test/test_time.py
index f224212..810ec37 100644
--- a/Lib/test/test_time.py
+++ b/Lib/test/test_time.py
@@ -126,6 +126,10 @@
             except ValueError:
                 self.fail('conversion specifier: %r failed.' % format)
 
+        self.assertRaises(TypeError, time.strftime, b'%S', tt)
+        # embedded null character
+        self.assertRaises(ValueError, time.strftime, '%S\0', tt)
+
     def _bounds_checking(self, func):
         # Make sure that strftime() checks the bounds of the various parts
         # of the time tuple (0 is valid for *all* values).