lib: fix NULL->field_len access

lib: fix NULL->field_len access

Currenlty, this function is used only by:
  - unit test of csv.c (see its main() section)
  - ptm_lib.c

In case of ptm, it is safe to return NULL because:
  csv_encode_record() -> return NULL
     _ptm_lib_encode_header() -> return NULL
        the only consumer of the return value is: ptm_lib_init_msg()
        that checks the NULL return.

Warning:
  Access to field 'field_len' results in a dereference of a null
  pointer (loaded from variable 'fld')

Signed-off-by: Vincent Jardin <vincent.jardin@6wind.com>