Problem:    No compiler warning for wrong format in vim_snprintf().
Solution:   Add printf attribute for gcc.  Fix reported problems.

Problem:    Crash in parsing JSON.
Solution:   Fail when using array or dict as dict key. (Damien)

Problem:    Encoding empty string to JSON sometimes gives "null".
Solution:   Handle NULL string as empty string. (closes #2446)

Problem:    Cannot use an empty key in json.
Solution:   Allow for using an empty key.

Problem:    When decoding JSON with a JS style object the JSON test may use a
            NULL pointer. (Coverity)
Solution:   Check for a NULL pointer.

Problem:    Error E937 is used both for duplicate key in JSON and for trying
            to delete a buffer that is in use.
Solution:   Rename the JSON error to E938. (Norio Takagi, closes #1376)

Problem:    JS style JSON does not support single quotes.
Solution:   Allow for single quotes. (Yasuhiro Matsumoto, closes #1371)

Problem:    Channel test fails for using freed memory.
Solution:   Fix memory use in json_decode().

Problem:    For complicated string json_decode() may run out of stack space.
Solution:   Change the recursive solution into an iterative solution.

Problem:    JSON with a duplicate key gives an internal error. (Lcd)
Solution:   Give a normal error.  Avoid an error when parsing JSON from a
            remote client fails.