fix unmatched nested comment edge case

Author: dxu

src/reason_lexer.mll

@@ -581,8 +581,8 @@ and comment = parse
           let loc = Location.curr lexbuf in
           raise (Error (Unmatched_nested_comment loc, start_loc))
         (* a multiline comment nested within a line comment *)
-        | (_, Some _) ->
-          comment_start_loc := [];
+        | (_ :: l, Some _) ->
+          comment_start_loc := l;
           store_lexeme lexbuf;
           comment lexbuf;
        }