…ckDB engine

Fixes alibaba#131

Description
===========
- Server crashes with assertion failure when executing:
  INSERT IGNORE INTO t (col1) VALUES (57399)
  where col1 is a DATETIME column on a DuckDB engine table.
- The assertion `mon > 0 && mon < 13 && year <= 9999` in sec_since_epoch()
  (sql/tztime.cc:356) fails because the MYSQL_TIME struct contains month=0.
- InnoDB handles this case gracefully by truncating to zero date with a warning,
  but DuckDB engine crashes.

Cause
=====
- MySQL's SQL layer converts the invalid integer 57399 to a zero date
  (0000-00-00 00:00:00) via number_to_datetime() -> reset(), setting month=0.
- In the DuckDB write path, DeltaAppender::append_mysql_field() for
  MYSQL_TYPE_DATETIME2 calls TIME_to_gmt_sec() without validating the
  MYSQL_TIME struct, which triggers the assertion in sec_since_epoch().

Fix
===
- Add a zero/invalid date check (tm.month == 0) in the MYSQL_TYPE_DATETIME2
  branch of DeltaAppender::append_mysql_field() before calling TIME_to_gmt_sec().
- When a zero date is detected, compute the timestamp directly using calc_daynr(),
  consistent with the existing MYSQL_TYPE_NEWDATE handling.
- Valid dates (month in [1, 12]) continue to use the original TIME_to_gmt_sec() path.