Bug#13011410 CRASH IN FILESORT CODE WITH GROUP BY/ROLLUP

The assert in 13580775 is visible in 5.6 only, 
but shows that all versions are vulnerable.
13011410 crashes in all versions.

filesort tries to re-use the sort buffer between invocations in order to save
malloc/free overhead.
The fix for Bug 11748783 - 37359: FILESORT CAN BE MORE EFFICIENT.
added an assert that buffer properties (num_records, record_length) are
consistent between invocations. Indeed, they are not necessarily consistent.
  
Fix: re-allocate the sort buffer if properties change.


mysql-test/r/partition.result:
  New tests.
mysql-test/t/partition.test:
  New tests.
sql/filesort.cc:
  If we already have allocated a sort buffer in a previous execution,
  then verify that it is big enough for the current one.
sql/table.h:
  Add sort_keys_size; Number of bytes allocated for the sort_keys buffer.