UP PROPERLY, MEMORY LEAKED

This memory leak happend if ALTER TABLE ... SET DEFAULT was
used to set a default value for a TINY|MEDIUM|LONG BLOB|TEXT
column.

According to the manual,
"BLOB and TEXT columns cannot be assigned a default value."
This was true for CREATE TABLE and ALTER TABLE ADD/MODIFY
COLUMN. However, for ALTER TABLE ... SET DEFAULT it was only
enforced for plain BLOB|TEXT columns and not for their 
tiny/medium/long variations.

When creating the record buffer for the default values of
blob columns (represented internally by Field_blob),
Field_blob::free() and ~Field_blob() were never called and
memory allocated for Field_blob::value was therefore never
released.

This patch fixes the problem by reporting
ER_BLOB_CANT_HAVE_DEFAULT error for TINY|MEDIUM|LONG BLOB|TEXT
columns, similarly to what is already done for BLOB|TEXT columns
and similar to what is already done for CREATE TABLE and
ALTER TABLE ADD/MODIFY COLUMN.

Note that this also disallows ALTER TABLE ... SET DEFAULT NULL
for geometry column types (Field_geom). Such columns are 
not supposed to support SET DEFAULT and everything but
SET DEFAULT NULL was already disallowed.