Always return NULL on error.
Adding also the file length.
  modified:   storage/connect/json.cpp
  modified:   storage/connect/jsonudf.cpp

- JSONCOL::WriteColumn Add types SHORT and BIGINT as accepted
  modified:   storage/connect/tabjson.cpp

- TDBJSN: Make this type use a separate storage for Json parsing
and retrieve this memory between each rows. This is necessary
to be able to handle big tables. See MDEV-9228.
  modified:   storage/connect/tabjson.cpp
  modified:   storage/connect/tabjson.h