IndexOutOfBoundsException in RelDecorrelator.decorrelateQuery

This query on the builtin example "wikipedia" dataset:

```
SELECT
(SELECT MIN(__time) FROM wikipedia w2 WHERE w1.countryName = w2.countryName) AS firstseen,
countryName
FROM wikipedia w1
GROUP BY countryName
ORDER BY firstseen
```

Throws the following error during planning:

```
java.lang.IndexOutOfBoundsException: index (6) must be less than size (2)
at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:313) ~[guava-16.0.1.jar:?]
at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:295) ~[guava-16.0.1.jar:?]
at com.google.common.collect.RegularImmutableList.get(RegularImmutableList.java:65) ~[guava-16.0.1.jar:?]
at org.apache.calcite.rex.RexBuilder.makeInputRef(RexBuilder.java:859) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.sql2rel.RelDecorrelator$AdjustProjectForCountAggregateRule.lambda$onMatch2$0(RelDecorrelator.java:2501) ~[calcite-core-1.21.0.jar:1.21.0]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[?:1.8.0_252]
at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[?:1.8.0_252]
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[?:1.8.0_252]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[?:1.8.0_252]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[?:1.8.0_252]
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_252]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_252]
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566) ~[?:1.8.0_252]
at org.apache.calcite.sql2rel.RelDecorrelator$AdjustProjectForCountAggregateRule.onMatch2(RelDecorrelator.java:2502) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.sql2rel.RelDecorrelator$AdjustProjectForCountAggregateRule.onMatch(RelDecorrelator.java:2433) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.AbstractRelOptPlanner.fireRule(AbstractRelOptPlanner.java:319) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepPlanner.applyRule(HepPlanner.java:560) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepPlanner.applyRules(HepPlanner.java:419) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepPlanner.executeInstruction(HepPlanner.java:256) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepInstruction$RuleInstance.execute(HepInstruction.java:127) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepPlanner.executeProgram(HepPlanner.java:215) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.plan.hep.HepPlanner.findBestExp(HepPlanner.java:202) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.sql2rel.RelDecorrelator.decorrelate(RelDecorrelator.java:250) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.sql2rel.RelDecorrelator.decorrelateQuery(RelDecorrelator.java:215) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.druid.sql.calcite.planner.Rules$DecorrelateAndTrimFieldsProgram.run(Rules.java:300) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.calcite.tools.Programs$SequenceProgram.run(Programs.java:346) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.tools.Programs$SequenceProgram.run(Programs.java:346) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.calcite.prepare.PlannerImpl.transform(PlannerImpl.java:357) ~[calcite-core-1.21.0.jar:1.21.0]
at org.apache.druid.sql.calcite.planner.DruidPlanner.planWithDruidConvention(DruidPlanner.java:205) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.druid.sql.calcite.planner.DruidPlanner.plan(DruidPlanner.java:139) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.druid.sql.SqlLifecycle.plan(SqlLifecycle.java:168) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.druid.sql.SqlLifecycle.plan(SqlLifecycle.java:179) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.druid.sql.SqlLifecycle.planAndAuthorize(SqlLifecycle.java:240) ~[druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
at org.apache.druid.sql.http.SqlResource.doPost(SqlResource.java:95) [druid-sql-0.19.0-iap1.jar:0.19.0-iap1]
```

Seen in an internal version that is based on 0.19.0.

Run the supplied query against the builtin wikipedia dataset to reproduce the planning failure. Start at RelDecorrelator.decorrelateQuery and the AdjustProjectForCountAggregateRule in the stack trace, then trace the Druid planner entry points in Rules.java and DruidPlanner.java. Done means the query plans without IndexOutOfBoundsException and regression coverage preserves that behavior.

Written by the indexing model from the issue text.