Add regression test for issue 953

Author: dtolnay

tests/regression/issue953.rs

@@ -0,0 +1,9 @@
+use serde_json::Value;
+
+#[test]
+fn test() {
+    let x1 = serde_json::from_str::<Value>("18446744073709551615.");
+    assert!(x1.is_err());
+    let x2 = serde_json::from_str::<Value>("18446744073709551616.");
+    assert!(x2.is_err());
+}