readline: double free when using PageDown

[Matthias Klose]

[forwarded from https://bugs.debian.org/1056314]

"""
I am getting a crash in the Python REPL in this scenario:

1. start "python3" in a terminal
2. type "2+2", enter
3. type (or copy-paste) "1234+5678", enter
4. arrow-up, remove "234", page-down, arrow-up, enter
5. arrow-up, arrow-up, add "000" (or whatever) after the 1, enter.

this results in:

free(): double free detected in tcache 2
 [1]    2319820 IOT instruction  python3
"""

this can be reproduced as well when entering these in bash.
readline is 8.2 patchlevel 7, bash is 5.2 patchlevel 21.