fix: avoid panic when parsing with a multi-byte delimiter

Ini::parse split the value using delimiter + 1, assuming the delimiter
occupies a single byte. With a multi-byte UTF-8 delimiter this offset lands
inside the delimiter character, and slicing the string there panics. Advance
by the delimiter's actual UTF-8 length instead.

src/ini.rs

@@ -1017,7 +1017,13 @@ impl Ini {
                                 current_key = Some(key.clone());
                             }
 
-                            let value = trimmed[delimiter + 1..].trim().to_owned();
+                            // `delimiter` is a byte offset and may itself be multi-byte, so advance by
+                            // its UTF-8 length.
+                            let delimiter_len = trimmed[delimiter..]
+                                .chars()
+                                .next()
+                                .map_or(1, char::len_utf8);
+                            let value = trimmed[delimiter + delimiter_len..].trim().to_owned();
 
                             valmap.insert(key, Some(value));
                         }

tests/test.rs

@@ -973,3 +973,18 @@ fn serde_case_sensitive_roundtrip() -> Result<(), Box<dyn Error>> {
 
     Ok(())
 }
+
+#[test]
+fn multibyte_delimiter_does_not_panic() -> Result<(), Box<dyn Error>> {
+    use configparser::ini::IniDefault;
+
+    // A multi-byte delimiter must not split on a non-char boundary.
+    let mut defaults = IniDefault::default();
+    defaults.delimiters = vec!['\u{00a7}']; // section sign, 2 bytes in UTF-8
+
+    let mut config = Ini::new_from_defaults(defaults);
+    config.read(String::from("[s]\nkey\u{00a7}value\n"))?;
+
+    assert_eq!(config.get("s", "key"), Some(String::from("value")));
+    Ok(())
+}