Compressed blob columns didn't accept data at their capacity. E.g. storing
255 bytes to TINYBLOB results in "Data too long" error.

Now it is allowed assuming compression method was able to produce shorter
string (so that both metadata and compressed data fits blob) and
column_compression_threshold is lower than blob.

If no compression was performed, we still have to reserve additional byte
for metadata and thus we perform normal data truncation and return it's
status.