Problem was when calculating the range of partitions for
pruning.

Solution was to get the calculation correct. I also simplified
it a bit for easier understanding.

mysql-test/r/partition_pruning.result:
  Bug#49742: Partition Pruning not working correctly for RANGE
  
  Added results.
mysql-test/t/partition_pruning.test:
  Bug#49742: Partition Pruning not working correctly for RANGE
  
  Added tests to prevent regressions.
sql/sql_partition.cc:
  Bug#49742: Partition Pruning not working correctly for RANGE
  
  Simplified calculation for partition id for ranges.
  Easier to get right and understand.
  
  Added comments.