SWIG_AsWCharPtrAndSize improper operation if cptr NULL

SF bug #1327

This doesn't have any noticeable effect with the usage of
SWIG_AsWCharPtrAndSize as shipped by SWIG, but could be a problem if a
user is using this function with cptr equal to zero and psize is non-zero
 - the length would be incorrectly set due to the call to PyUnicode_GetSize
failing.