Make ExecGetInsertedCols() and friends more robust and improve comments.

 Make ExecGetInsertedCols() and friends more robust and improve comments.
 
 If ExecGetInsertedCols(), ExecGetUpdatedCols() or ExecGetExtraUpdatedCols()
 were called with a ResultRelInfo that's not in the range table and isn't a
 partition routing target, the functions would dereference a NULL pointer,
 relinfo->ri_RootResultRelInfo. Such ResultRelInfos are created when firing
 RI triggers in tables that are not modified directly. None of the current
 callers of these functions pass such relations, so this isn't a live bug,
 but let's make them more robust.
 
 Also update comment in ResultRelInfo; after commit 6214e2b228,
 ri_RangeTableIndex is zero for ResultRelInfos created for partition tuple
 routing.
 
 Noted by Coverity. Backpatch down to v11, like commit 6214e2b228.
 
 Reviewed-by: Tom Lane, Amit Langote