msvcrt: Fix _wdupenv_s behavior with missing env var.

dlls/msvcr80/tests/msvcr80.c

@@ -211,11 +211,8 @@ static void test_wdupenv_s(void)
     len = 0xdeadbeef;
     tmp = (void *)0xdeadbeef;
     ret = p_wdupenv_s( &tmp, &len, L"nonexistent" );
-    todo_wine
     ok( !ret, "_wdupenv_s returned %d\n", ret );
-    todo_wine
     ok( !len, "_wdupenv_s returned length is %Id\n", len );
-    todo_wine
     ok( !tmp, "_wdupenv_s returned pointer is %p\n", tmp );
 }
 

dlls/msvcrt/environ.c

@@ -279,7 +279,12 @@ int CDECL _wdupenv_s(wchar_t **buffer, size_t *numberOfElements,
     if (!MSVCRT_CHECK_PMT(buffer != NULL)) return EINVAL;
     if (!MSVCRT_CHECK_PMT(varname != NULL)) return EINVAL;
 
-    if (!(e = _wgetenv(varname))) return *_errno() = EINVAL;
+    if (!(e = _wgetenv(varname)))
+    {
+        *buffer = NULL;
+        if (numberOfElements) *numberOfElements = 0;
+        return 0;
+    }
 
     sz = wcslen(e) + 1;
     if (!(*buffer = malloc(sz * sizeof(wchar_t))))