[PATCH v2] user32/caret.c: Avoid SetSystemTimer / KillSystemTimer with null hwnd.

[Robert Schlicht]

This patch (ID 178871) has been superseded by the patch with ID 179405
[PATCH] server: Make set_caret_info fail if there is no caret window.

Signed-off-by: Robert Schlicht <patch(a)rschlicht.eu>