TIMESTAMP field when no value has been provided.

The LOAD DATA sets the current time in the TIMESTAMP field with
CURRENT_TIMESTAMP default value when the field is detected as a null.
But when the LOAD DATA command loads data from a file that doesn't contain
enough data for all fields then the rest of fields are simply set to null
without any check. This leads to no value being inserted to such TIMESTAMP
field.

Now the read_sep_field() and the read_fixed_length() functions set current
time to the TIMESTAMP field with CURRENT_TIMESTAMP default value in all cases
when a NULL value is loaded to the field.