[PATCH] user32: Make sure that passed window handle is scroll control before accessing wExtra in SCROLL_GetInternalInfo.

[Jacek Caban]

Fixes memory corruption in Office 2016.

Signed-off-by: Jacek Caban <jacek(a)codeweavers.com>
---

I'm not sure a new window flag is the best way to handle it. I did it
the same way as it's done for MDI client.

 dlls/user32/scroll.c       |  7 ++++++-
 dlls/user32/tests/scroll.c | 21 +++++++++++++++++++++
 dlls/user32/win.h          |  1 +
 3 files changed, 28 insertions(+), 1 deletion(-)