Fixes pretty_contents undefined

pre_commit_hooks/pretty_format_json.py

@@ -105,6 +105,7 @@ def main(argv: Sequence[str] | None = None) -> int:
         with open(json_file, encoding='UTF-8') as f:
             contents = f.read()
 
+        is_error = False
         try:
             pretty_contents = _get_pretty_format(
                 contents, args.indent, ensure_ascii=not args.no_ensure_ascii,
@@ -115,9 +116,10 @@ def main(argv: Sequence[str] | None = None) -> int:
                 f'Input File {json_file} is not a valid JSON, consider using '
                 f'check-json',
             )
+            is_error = True
             status = 1
 
-        if contents != pretty_contents:
+        if contents != pretty_contents and not is_error:
             if args.autofix:
                 _autofix(json_file, pretty_contents)
             else: