rust: replace handwritten str_to_cstr_vec with alloc::ffi::CString

alloc::ffi::CString was added in Rust 1.64 and can be used as a drop
in to our handwritten function.

Author: benma

src/rust/bitbox02/src/util.rs

@@ -105,15 +105,9 @@ pub fn truncate_str(s: &str, len: usize) -> &str {
 /// Converts a Rust string to a null terminated C string by appending a null
 /// terminator.  Returns `Err(())` if the input already contians a null byte.
 pub fn str_to_cstr_vec(input: &str) -> Result<Vec<u8>, ()> {
-    let bytes = input.as_bytes();
-    if bytes.contains(&0) {
-        Err(())
-    } else {
-        let mut out = Vec::with_capacity(input.len() + 1);
-        out.extend_from_slice(bytes);
-        out.push(0); // null terminator
-        Ok(out)
-    }
+    Ok(alloc::ffi::CString::new(input)
+        .or(Err(()))?
+        .into_bytes_with_nul())
 }
 
 #[cfg(test)]