From the project-root:

0. cd src
1. gdb ./emacs
2. set logging on
3. r -Q --fg-daemon

From the project-root in another shell:

4. ./lib-src/emacsclient -c
5. C-\ arabic RET
6. s ~ A
7. C-x 5 0
8. ./lib-src/emacsclient -c

Back to GDB:

9. bt full

I attach the resulting backtrace.

I then recompiled ftcrfont.c with the following printf sprinkles: