Added a test case for bug #11295.
item_buff.cc:
Fixed bug #11295.
This a correction for the patch of bug #11088 that takes into
account a possible NULL values of the BLOB column.
Fixed bug #11088: a crash for queries with GROUP BY a BLOB column
+ COUNT(DISTINCT...) due to an attempt to allocate a too large
buffer for the BLOB field.
Now the size of the buffer is limited by max_sort_length.
group_by.test, group_by.result:
Added a test case for bug #11088.