Fix seg fault using Python 2 invalid utf-8 strings and wstring

Fixes seg fault when passing a Python string, containing invalid utf-8 content,
to a wstring or wchar * parameter.  A TypeError is thrown instead, eg:

  %include <std_wstring.i>
  void instring(const std::wstring& s);

  instring(b"h\xe9llooo") # Python

Examples/test-suite/li_std_wstring.i

@@ -94,6 +94,14 @@ void test_throw() TESTCASE_THROW1(std::wstring){
   throw x;
 }
 
+const char * non_utf8_c_str() {
+  return "h\xe9llo";
+}
+
+size_t size_wstring(const std::wstring& s) {
+  return s.size();
+}
+
 #ifdef SWIGPYTHON_BUILTIN
 bool is_python_builtin() { return true; }
 #else