Incorrect vartable compaction to TINY format.

Var-table compaction may lead to row data corruption when TINY format is selected
in case of 256+ empty (not null) varlen fields.

TINY format will be selected in this case, because payload size fit to a single byte,
however, the size of vartable will be truncated{{ int->byte}} with overflow.

Possible fixes are:

• switch to MEDIUM format
• always use short for vartable size