fix: reproduce and fix panic when parsing integer values

Previously it would try to treat a UTF-8 string like ASCII and split
the last character off, even though it wasn't always at a charcter boundary.

Author: Byron

gix-config-value/src/integer.rs

@@ -67,6 +67,11 @@ impl TryFrom<&BStr> for Integer {
             return Err(int_err(s));
         }
 
+        let last_idx = s.len() - 1;
+        if !s.is_char_boundary(last_idx) {
+            return Err(int_err(s));
+        }
+
         let (number, suffix) = s.split_at(s.len() - 1);
         if let (Ok(value), Ok(suffix)) = (number.parse(), suffix.parse()) {
             Ok(Self {

gix-config-value/tests/value/integer.rs

@@ -53,6 +53,7 @@ fn invalid_from_str() {
     assert!(Integer::try_from(b("g")).is_err());
     assert!(Integer::try_from(b("123123123123123123123123")).is_err());
     assert!(Integer::try_from(b("gg")).is_err());
+    assert!(Integer::try_from(b("™️🤦‍♂️")).is_err());
 }
 
 #[test]