to 0 causes wrong (large) length to be read
 from the row in _mi_calc_blob_length() when 
 storing NULL values in (e.g) POINT columns.
 This large length is then used to allocate
 a block of memory that (on some OSes) causes
 trouble.
 Fixed by calling the base class's 
 Field_blob::reset() from Field_geom::reset()
 that is called when storing a NULL value into
 the column.