As we check for the impossible partitions earlier, it's possible that we don't find any
              suitable partitions at all. So this assertion just has to be corrected for this case.
      
per-file comments:
mysql-test/r/partition_innodb.result
Bug#55146      Assertion `m_part_spec.start_part == m_part_spec.end_part' in index_read_idx_map
        test result updated.

mysql-test/t/partition_innodb.test
Bug#55146      Assertion `m_part_spec.start_part == m_part_spec.end_part' in index_read_idx_map
        test case added.

sql/ha_partition.cc
Bug#55146      Assertion `m_part_spec.start_part == m_part_spec.end_part' in index_read_idx_map
        Assertion changed to '>=' as the  prune_partition_set() in the get_partition_set() can
        do start_part= end_part+1 if no possible partitions were found.