MDEV-41064 Use my_safe_alloca for unescaped json server option string

[mariadb-YuchenPei]

This fixes segv when the option string is longer than thread stack

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}