gh-123321: Fix Parser/myreadline.c to prevent a segfault during a multi-threaded race #123455

Job	Run time
Change detection / Compute changed files	10s
Ubuntu SSL tests with OpenSSL (1.1.1w)	2m 25s
Ubuntu SSL tests with OpenSSL (3.0.13)	2m 20s
Windows MSI${{ '' }}	0s
Check if generated files are up to date	2m 22s
Address sanitizer	2m 23s
Ubuntu SSL tests with OpenSSL (3.1.5)	2m 23s
Ubuntu SSL tests with OpenSSL (3.2.1)	2m 16s
CIFuzz (address)	2m 10s
Hypothesis tests on Ubuntu	2m 25s
CIFuzz (undefined)	2m 13s
CIFuzz (memory)	2m 11s
WASI / build and test	2m 24s
Thread sanitizer / Thread sanitizer	2m 22s
Thread sanitizer (free-threading) / Thread sanitizer	2m 23s
Windows (free-threading) / build (arm64)	2m 1s
Windows / build (arm64)	2m 6s
Windows / build and test (Win32)	2m 8s
Windows (free-threading) / build and test (Win32)	2m 8s
macOS (free-threading) / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	2m 10s
macOS / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	2m 14s
macOS / build and test (macos-13)	2m 25s
Ubuntu / build and test	2m 22s
Ubuntu (free-threading) / build and test	2m 22s
Windows / build and test (x64)	2m 7s
Windows (free-threading) / build and test (x64)	2m 8s
Docs / Docs	1m 48s
Docs / Doctest	2m 21s
Docs / Docs (Oldest Sphinx)	1m 59s
All required checks pass	4s
	1h 0m 50s

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