I ran the random_fuzzer.sh script on a freebsd14.2 amd64 virtual machine after @clausecker reported a crash. I was not able to reproduce that particular crash, but I was able to make one on my own.

I was not able to reproduce this on debian (using clang 18.1.8) using the same flags.

the input data

• (base64 encoded): IgEBELw=
• (hex): 22 01 01 10 bc

This means the convert_utf8_to_latin1_with_errors() is invoked with the single byte 0xbc

fuzzer: conversion
operating system: freebsd 14.2-release
compiler: clang 18.1.6
flags: -fsanitize=fuzzer-no-link,address,undefined -g -O0 -fsanitize-trap=undefined

the error message is (sorry for the briefness, had to write this by hand, currently no copy paste support from the virtual machine):

ERROR: AddressSanitizer: BUS on unknown address
The signal is caused by a READ memory access.
Hint: this fault was caused by a dereference of a high value address 
icelake_convert_utf8_to_latin1.inl.cpp:85:22

This is the place in the code which is pointed to: