[3.13] gh-123321: Fix Parser/myreadline.c to prevent a segfault during a multi-threaded race (GH-123323) (#123676)

gh-123321: Fix Parser/myreadline.c to prevent a segfault during a multi-threaded race (GH-123323)
(cherry picked from commit a4562fedadb73fe1e978dece65c3bcefb4606678)

Co-authored-by: Bar Harel <bharel@barharel.com>