fix(errors): adjust column offset if not at char boundary

Fixes ijl#226

See https://www.reddit.com/r/rust/comments/765n76/psa_caution_with_slicing_strings/

src/deserialize/error.rs

@@ -41,7 +41,13 @@ impl<'a> DeserializeError<'a> {
                     //       directly use the `column` field
                     if self.column == 0 { return 0; }
 
-                    let chars_count = s[..self.column - 1].chars().count();
+                    // Find a column we can safely slice on
+                    let mut column = self.column - 1;
+                    while column > 0 && !s.is_char_boundary(column) {
+                        column -= 1;
+                    }
+
+                    let chars_count = s[..column].chars().count();
                     if chars_count == s.chars().count() - 1 {
                         chars_count + 1
                     } else {

test/test_issue221.py

@@ -0,0 +1,11 @@
+import pytest
+import orjson
+
+
+@pytest.mark.parametrize("input", [
+    b'"\xc8\x93',
+    b'"\xc8',
+])
+def test_invalid(input):
+    with pytest.raises(orjson.JSONDecodeError):
+        orjson.loads(input)