HPCC-2998 Remove getRecordSize(NULL) from result reading

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-2998 Move helper to thorcommon and delete unused class

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4117 from ghalliday/issue2998b

HPCC-2998 Remove getRecordSize(NULL) from result reading

Reviewed-By: Richard Chapman <rchapman@hpccsystem.com>