Bug#11764126 SUM(DISTINCT) GIVES WRONG RESULT WHEN REDUCING MAX_HEAP_TABLE_SIZE

MySQL server creates in-memory data structures in order to execute
queries of the form
  SELECT SUM(DISTINCT col) sm FROM tab;
The size of those data structures is limited by the value of the
system variable max_heap_table_size.

If the table contains many distinct values, intermediate data will be
written to disk, and later re-merged. There was a bug in this
merge algorithm, which may lead to incorrect results.
The workaround was to increase the value of max_heap_table_size.

This patch fixes the merge algorithm.