Problem: Field_geom (which is a Field_blob child) did not use
table->blob_storage - a memory helper storage previously added into Field_blob,
to handle "GROUP_CONCAT with DISTINCT or ORDER BY" properly.

Fix:
- Moving table->blob_storage related code into a separate private
  method Field_blob::store_to_mem().

- Moving Field_blob and Field_geom specific code into a new private
  virtual methods Field_blob::store_internal() and Field_geom::store_internal().

- Rewriting Field_blob::store(), so now it shares the common code
  between Field_blob and Field_geom, to handle empty strings and
  table->blob_storage operations, and then calls the specific code
  from store_internal().

- An additional helper protected  method Field_blob::store_ptr_and_length()
  was added, to avoid a few duplicate code pieces in Field_blob and
  Field_geom.