gh-123321: Fix Parser/myreadline.c to prevent a segfault during a multi-threaded race #123469

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2 errors and 3 warnings

Canceling since a higher priority waiting request for 'Tests-gh-123321-reusable' exists

The operation was canceled.

pkg-config 0.29.2_3 is already installed and up-to-date. To reinstall 0.29.2_3, run: brew reinstall pkg-config

xz 5.6.2 is already installed and up-to-date. To reinstall 5.6.2, run: brew reinstall xz

HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK is set: not checking for outdated dependents or dependents with broken linkage!