HPCC-11846 Fix various clang warnings

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
This one is interesting.  The members are initialised in the order they are declared - which means start is initialised after whichNext - so whichNext(start) assigns an undefined value.  As long as everyone calls first() it isn't going to cause any problems.