debug_assert_eq! in BatchCoalescer panics in debug mode when batch_size < 4 #9506

  use arrow::compute::BatchCoalescer;                                                           
  use arrow::datatypes::{DataType, Field, Schema};                                              
  use arrow::array::{Int32Array, RecordBatch};                                             
  use std::sync::Arc;                                                                           
                                                                                              
  let schema = Arc::new(Schema::new(vec![Field::new("a", DataType::Int32, false)]));            
  let mut coalescer = BatchCoalescer::new(schema.clone(), 2);                                   
  let batch = RecordBatch::try_new(
      schema,
      vec![Arc::new(Int32Array::from(vec![1, 2, 3]))],
  ).unwrap();
  coalescer.push_batch(batch).unwrap(); // panics in debug mode