Segmentation fault in function getprop_builtin_string (at mjs.c:8421)

[Vancir]

I found a segmentation fault bug while mjs (latest, b1b6eac) executes the PoC on Ubuntu 20.04.

Download the PoC: mjs_segv_mjs8421.zip

ASAN Report

==3004153==ERROR: AddressSanitizer: SEGV on unknown address 0x0000004f40b9 (pc 0x0000004f40b9 bp 0x7fff52e4b110 sp 0x7fff52e4adf8 T0)
==3004153==The signal is caused by a WRITE memory access.
    #0 0x4f40b9 in getprop_builtin_string /data/song/projects/latest-programs/mjs/BUILD/mjs.c:8421:14
    #1 0x4dd8ee in mjs_exec_internal /data/song/projects/latest-programs/mjs/BUILD/mjs.c:9044:5
    #2 0x4ddaeb in mjs_exec_file /data/song/projects/latest-programs/mjs/BUILD/mjs.c:9067:11
    #3 0x4e1df7 in main /data/song/projects/latest-programs/mjs/BUILD/mjs.c:11406:13
    #4 0x7f320b4dad8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #5 0x7f320b4dae3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #6 0x41f344 in _start (/data/song/projects/latest-programs/mjs/mjs-bin+0x41f344)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /data/song/projects/latest-programs/mjs/BUILD/mjs.c:8421:14 in getprop_builtin_string

Steps to reproduce

git clone https://github.com/cesanta/mjs.git
cd mjs
clang -g -O1 -fsanitize=address -fno-omit-frame-pointer -Wno-error -DMJS_MAIN mjs.c -dl -o mjs_asan
./mjs_asan -f PoC