Aliased blob variable with less restrictions makes text blob accept malformed string through BLOB_APPEND

[asfernandes]

-- Wrong: no "Malformed string" error
execute block
as
    declare b blob sub_type text character set utf8;
    declare bb blob sub_type binary;
begin
    b = blob_append(b, 'a');
    bb = b;
    -- No error
    b = blob_append(bb, x'FF');
    -- Malformed blob is saved to table
    insert into t (b_utf8) values (:b);
end!