gh-125966: fix UAF on `fut->fut_callback0` due to an evil callback's `eq` #130568

Job	Run time
Change detection / Compute changed files	14s
Check if Autoconf files are up to date	25s
Check if generated files are up to date	3m 39s
Address sanitizer	7m 43s
Ubuntu SSL tests with OpenSSL (ubuntu-22.04, 3.0.15)	2m 9s
Ubuntu SSL tests with OpenSSL (ubuntu-22.04, 3.1.7)	2m 25s
Windows MSI${{ '' }}	0s
CIFuzz (address)	19m 53s
Ubuntu SSL tests with OpenSSL (ubuntu-22.04, 3.2.3)	2m 19s
CIFuzz (undefined)	21m 43s
Ubuntu SSL tests with OpenSSL (ubuntu-22.04, 3.3.2)	2m 5s
CIFuzz (memory)	20m 44s
Hypothesis tests on Ubuntu	5m 23s
Thread sanitizer (free-threading) / Thread sanitizer	7m 31s
Thread sanitizer / Thread sanitizer	2m 54s
WASI / build and test	5m 56s
Windows / build (arm64)	3m 6s
Windows (free-threading) / build (arm64)	3m 5s
Windows / build and test (Win32)	16m 32s
Windows / build and test (x64)	15m 49s
Windows (free-threading) / build and test (Win32)	24m 30s
Windows (free-threading) / build and test (x64)	23m 31s
macOS (free-threading) / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	5m 3s
macOS / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	4m 39s
macOS / build and test (macos-13)	12m 12s
Ubuntu / build and test (ubuntu-22.04)	7m 9s
Ubuntu (free-threading) / build and test (ubuntu-22.04)	9m 16s
Docs / Docs	2m 10s
Docs / Docs (Oldest Sphinx)	2m 4s
Docs / Doctest	30m 25s
All required checks pass	1s
	4h 24m 35s

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