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

[Marvin]

Hi,

While running your changed tests, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?

Full results can be found at:
https://testbot.winehq.org/JobDetails.pl?Key=65200

Your paranoid android.


=== debian10 (32 bit Chinese:China report) ===

user32:
msg.c:17559: Test succeeded inside todo block: WaitForSingleObject failed, ret:0
msg.c:17569: Test failed: wrong status 00080000
msg.c:17583: Test succeeded inside todo block: wrong status 00000000
msg.c:17587: Test succeeded inside todo block: SendMessage from other thread 5: marked "todo_wine" but succeeds