user32: Fix return value in ShowScrollBar according with test.

dlls/user32/scroll.c

@@ -1998,6 +1998,9 @@ static BOOL SCROLL_ShowScrollBar( HWND hwnd, INT nBar, BOOL fShowH, BOOL fShowV
  */
 BOOL WINAPI ShowScrollBar(HWND hwnd, INT nBar, BOOL fShow)
 {
+    if ( !hwnd )
+        return FALSE;
+
     SCROLL_ShowScrollBar( hwnd, nBar, (nBar == SB_VERT) ? 0 : fShow,
                                       (nBar == SB_HORZ) ? 0 : fShow );
     return TRUE;

dlls/user32/tests/scroll.c

@@ -123,10 +123,7 @@ static void scrollbar_test3(void)
     ok( !IsWindowVisible( hScroll ), "The scrollbar window should be visible\n" );
 
     ret = ShowScrollBar( NULL, SB_CTL, TRUE );
-    todo_wine
-    {
-        ok( !ret, "The ShowScrollBar() should failed.\n" );
-    }
+    ok( !ret, "The ShowScrollBar() should failed.\n" );
 
 }