shmem::unix_shmem::MmapShMem::new crash on small rand_id

[anarcheuz]

Describe the bug
It seems that for smaller rand_id shmem::unix_shmem::MmapShMem::new() can crash when slicing. The shmem path is computed like this:

                    let full_file_name = format!("/libafl_{}_{}", process::id(), rand_id);
                    let mut filename_path = [0_u8; MAX_MMAP_FILENAME_LEN];
                    filename_path
                        .copy_from_slice(&full_file_name.as_bytes()[..MAX_MMAP_FILENAME_LEN]);

MAX_MMAP_FILENAME_LEN being 20, it is possible for full_file_name (at https://docs.rs/libafl_bolts/latest/src/libafl_bolts/shmem.rs.html#695) to be smaller than that and crash as shown in the stack trace below.

To Reproduce

The content of my cargo.toml:

[package]
name = "fd_fuzz"
version = "0.1.0"
edition = "2021"

[dependencies]
libafl = "0.13.2"
libafl_bolts = "0.13.2"

use libafl_bolts::shmem::MmapShMem;

fn main() {
    let shmem = MmapShMem::new(1, 1);
}

then run with RUST_BACKTRACE=1 cargo run.

Expected behavior
Call should not crash.

Screen output/Screenshots

thread 'main' panicked at /home/.../.cargo/registry/src/index.crates.io-6f17d22bba15001f/libafl_bolts-0.13.2/src/shmem.rs:695:68:
range end index 20 out of range for slice of length 16
stack backtrace:
   0: rust_begin_unwind
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/std/src/panicking.rs:662:5
   1: core::panicking::panic_fmt
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:74:14
   2: core::slice::index::slice_end_index_len_fail_rt
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/slice/index.rs:64:5
   3: core::slice::index::slice_end_index_len_fail
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/slice/index.rs:57:5
   4: <core::ops::range::Range<usize> as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/slice/index.rs:467:13
   5: <core::ops::range::RangeTo<usize> as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/slice/index.rs:553:9
   6: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/slice/index.rs:16:9
   7: libafl_bolts::shmem::unix_shmem::default::MmapShMem::new
             at /home/anthony/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libafl_bolts-0.13.2/src/shmem.rs:695:68
   8: fd_fuzz::main
             at ./src/main.rs:83:17
   9: core::ops::function::FnOnce::call_once
             at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/ops/function.rs:250:5

Additional context
Add any other context about the problem here.