Fix 'arithmetic operation overflowed' panic in json::Parser::decode_hex_escape

The overflow was introduced accidentally in 49d4958 and happened to cancel out.

Author: SimonSapin

src/json.rs

@@ -1585,7 +1585,7 @@ impl<T: Iterator<Item = char>> Parser<T> {
 
     fn decode_hex_escape(&mut self) -> Result<u16, ParserError> {
         let mut i = 0;
-        let mut n = 016;
+        let mut n = 0;
         while i < 4 && !self.eof() {
             self.bump();
             n = match self.ch_or_null() {