[3.6] bpo-30708: Check for null characters in PyUnicode_AsWideCharString(). (GH-2285...

[3.6] bpo-30708: Check for null characters in PyUnicode_AsWideCharString(). (GH-2285) (#2443)

Raise a ValueError if the second argument is NULL and the wchar_t\*
string contains null characters..
(cherry picked from commit e613e6add5f07ff6aad5802924596b631b707d2a)