Undefined behavior in `bedtools expand`

[SoapGentoo]

Reporting this from https://bugs.gentoo.org/896172:
compiling bedtools with CPPFLAGS="-D_GLIBCXX_ASSERTIONS=1" and running tests:

Testing bedtools expand:
    expand.t1.../usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/basic_string.h:1229: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]: Assertion '__pos <= size()' failed.
test-expand.sh: line 33: 421609 Aborted                 (core dumped) $BT expand -i expand.txt -c 4 > obs