multi: silence gcc 16 -Wnull-dereference, bump CI job to test

[vszakats]

• GHA/windows: bump dl-mingw job from gcc 15 to 16.

• multi: silence warning while building libcurlu:

  In function 'multi_ischanged',
      inlined from 'multi_socket.isra' at D:/a/curl/curl/lib/multi.c:3282:6:
  D:/a/curl/curl/lib/multi.c:1710:17: error: null pointer dereference [-Werror=null-dereference]
   1710 |   bool retval = (bool)multi->recheckstate;
        |                 ^~~~~~~~~~~~~~~~~~~~~~~~~
  D:/a/curl/curl/lib/multi.c:1712:25: error: null pointer dereference [-Werror=null-dereference]
   1712 |     multi->recheckstate = FALSE;
        |                         ^
  

  Ref: https://github.com/curl/curl/actions/runs/26217071531/job/77142119137?pr=21707

• multi: silence another -Wnull-dereference, popping up in libcurl
  with gcc 13 after the previous silencing:

  In function 'Curl_multi_xfers_running',
      inlined from 'multi_socket.isra' at ../../lib/multi.c:3292:28:
  ../../lib/multi.c:4132:15: error: null pointer dereference [-Werror=null-dereference]
   4132 |   return multi->xfers_alive;
        |          ~~~~~^~~~~~~~~~~~~
  

  Ref: https://github.com/curl/curl/actions/runs/26218822231/job/77148186045

• multi: also add DEBUGASSERT(multi) to the two updated functions.

[Copilot]

Pull request overview

This PR updates libcurl’s multi interface implementation and Windows CI to address GCC -Wnull-dereference warnings (treated as errors) and to ensure CI exercises the newer compiler/toolchain where the warnings occur.

Changes:

• Guard multi_ischanged() and Curl_multi_xfers_running() against NULL to silence GCC -Wnull-dereference, and add DEBUGASSERT(multi) in both.
• Bump the Windows “dl-mingw” CI job toolchain to w64devkit v2.8.0 / GCC 16.1.0.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/multi.c	Adds DEBUGASSERT and NULL guards to silence GCC null-dereference warnings in inlined multi codepaths.
.github/workflows/windows.yml	Updates the w64devkit download/version/SHA used by the dl-mingw job to test with GCC 16.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.