During UPDATE, there are two places where update_generated_write_fields() will
be called. The first place is in function fill_record. The other one is
set_function_defaults.

For updating virtual BLOB generated column, we have to avoid the BLOB content
rewritten (Ref comments on keep_old_value()). Therefore, we use
keep_old_value() to record the old 'value' before update. In the first call of
update_generated_write_fields(), we keep the old 'value'. However, in the
second call, we free the old 'value' according to the current logic.  So after
the second calling, the old 'value' isn't kept actually. This is why such
a bug occurs.

The solution in this patch introduces a variable in Field_blob which marks
whether the old 'value' is kept. If the variable is TRUE, we will keep the old
'value'.  Otherwise, we will skip to keep the old 'value'. By this way, we can
avoid the rewritten even there are two times of calling on keep_old_value().
This variable will be set to true by update_generated_read_fields() when
reading the record containing a virtual generated BLOB field. It will be set
back to false when update_generated_write_fields() calls
Field_blob::keep_old_value().