unpack_fields() didn't expect NULL_LENGHT in the field's descriptions.
In this case we get NULL in the resulting string so cannot use
strdup_root to make a copy of it.
strdup_root changed with strmake_root as it's NULL-safe


sql-common/client.c:
  Bug #29494 Field packet with NULL fields crashes libmysqlclient
  
  strdup_root changed with strmake_root in unpack_fields()