When a MariaDB 10.0 DATETIME field with a custom length was defined, the
field offsets weren't calculated properly.
As there is no metadata for pre-10.1 DATETIME types with decimal
precision, the metadata (i.e. decimal count) needs to be gathered from the
CREATE TABLE statement. This information is then used to calculate the
correct field length when the value is decoded.
This change does not fix the incorrect interpretation of the old DATETIME
value. The converted values are still garbled due to the fact that the
value needs to be shifted out of the decimal format before it can be
properly converted.
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