dyncol_blob is a valid packed dynamic columns blob. Return value of 1 means the blob is valid, return value of 0 means it is not.
Rationale: Normally, one works with valid dynamic column blobs. Functions like COLUMN_CREATE, COLUMN_ADD, COLUMN_DELETE always return valid dynamic column blobs. However, if a dynamic column blob is accidentally truncated, or transcoded from one character set to another, it will be corrupted. This function can be used to check if a value in a blob field is a valid dynamic column blob.