Bug occurs in INSERT IGNORE ... SELECT ... ON DUPLICATE KEY UPDATE
statements, when SELECT returns duplicated values and UPDATE clause
tries to assign NULL values to NOT NULL fields.
NOTE: By current design MySQL server treats INSERT IGNORE ... ON
DUPLICATE statements as INSERT ... ON DUPLICATE with update of
duplicated records, but MySQL manual lacks this information.
After this fix such behaviour becomes legalized.

The write_record() function was returning error values even within
INSERT IGNORE, because ignore_errors parameter of
the fill_record_n_invoke_before_triggers() function call was
always set to FALSE. FALSE is replaced by info->ignore.