[SPARK-19512][BACKPORT-2.1][SQL] codegen for compare structs fails #16852

## What changes were proposed in this pull request?

Set currentVars to null in GenerateOrdering.genComparisons before genCode is called. genCode ignores INPUT_ROW if currentVars is not null and in genComparisons we want it to use INPUT_ROW.

## How was this patch tested?

Added test with 2 queries in WholeStageCodegenSuite

Author: Bogdan Raducanu <bogdan.rdc@gmail.com>

Closes #16875 from bogdanrdc/SPARK-19512-2.1.