Buffer overflow due to wrong key length in partitioning

Changed to the correct key_length function.


sql/opt_range.cc:
  Bug#31705 Partitions: crash if varchar length > 65530
  Problem: buffer overflow due to wrong key-length
  
  Fix: Using correct key_length function
mysql-test/r/partition_datatype.result:
  Bug#31705 Partitions: crash if varchar length > 65530
  New test-result case for testing all column types
  used in key-partitioning.
  
  (For verifying correct key-length use)
mysql-test/t/partition_datatype.test:
  Bug#31705 Partitions: crash if varchar length > 65530 
  New test case for testing all column types
  used in key-partitioning.
  
  (For verifying correct key-length used)