Fix dead key handling on Win32 (again)

[dkwingsmt]

This PR tries to fix dead key handling on Win32:

• Fix dead character crashes the channel handler.
• Fix dead key messages are redispatched, making the message a regular character message.

It fixes the problem raised in #28047 (comment).

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[gspencergoog]

[knopp]

@dkwingsmt, thank you for this! There is one issue that I noticed. Normal behavior in windows, when pressing the dead key twice is that it gets printed twice. I.e. ´´, with no dead key state afterwards. However pressing the dead key twice in Flutter prints the first dead key while the second dead key becomes active.

[dkwingsmt]

@knopp That's interesting to know! I'll take a look afterwards. Thank you.