Treat control characters in JSON strings as invalid

Treat control characters in JSON strings as invalid

* lisp/json.el (json-peek): Reduce to following-char.
(json-pop, json-read): Zero (null char) means end of file.
(json-read-escaped-char): Delimit URL properly.
(json-read-string): Signal error for ASCII control characters.
* test/lisp/json-tests.el (test-json-peek): Check for zero instead of
:json-eof symbol.
(test-json-read-string): New test for control characters in JSON
strings.