for blob column

ANALYZE TABLE <table> does not collect statistical data on min/max values
for BLOB columns of <table>. However these values can be added into
mysql.column_stats manually by executing proper statements.
Unfortunately this led to a memory leak because the memory allocated
for these values was never freed.
This patch provides the server with a function to free memory allocated
for min/max statistical values of BLOB types.

Temporarily changed the test case until MDEV-16711 is fixed as without
this fix the test case for MDEV-16757 did not fail only for 10.0.