Fix panic in variance GroupsAccumulator (#12615)

The bug was in the orginal implementation in #12095. This fixes the
issue and modify a test case such that it would have caught it.

Author: eejbyfeldt

datafusion/functions-aggregate/src/variance.rs

@@ -470,7 +470,7 @@ impl VarianceGroupsAccumulator {
 
         if let StatsType::Sample = self.stats_type {
             counts.iter_mut().for_each(|count| {
-                *count -= 1;
+                *count = count.saturating_sub(1);
             });
         }
         let nulls = NullBuffer::from_iter(counts.iter().map(|&count| count != 0));

datafusion/sqllogictest/test_files/aggregate.slt

@@ -567,13 +567,13 @@ set datafusion.sql_parser.dialect = 'Postgres';
 
 # csv_query_stddev_12
 query IR
-SELECT c2, var_samp(c12) FILTER (WHERE c12 > 0.90) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+SELECT c2, var_samp(c12) FILTER (WHERE c12 > 0.95) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
 ----
-1 0.000889240174
-2 0.000785878272
+1 0.000791243479
+2 0.000061521903
 3 NULL
 4 NULL
-5 0.000269544643
+5 NULL
 
 # Restore the default dialect
 statement ok