In select_describe(), make the String object that holds the value of 
"partitions" column to "own" the value buffer, so the buffer isn't
prematurely freed.
[this is the second attempt with review fixes]


mysql-test/r/partition_pruning.result:
  BUG#19684: Testcase
mysql-test/t/partition_pruning.test:
  BUG#19684: Testcase
sql/sql_select.cc:
  BUG#19684: Garbage in 'partitions' column in EXPLAIN output:
  - Added comment
  - Make the String object that holds the value of "partitions" column
    to "own" the value buffer, so the buffer isn't prematurely freed.