NO_AUTO_VALUE_ON_ZERO mode.

The table->auto_increment_field_not_null variable wasn't reset after
reading a row which may lead to inserting a wrong value to the auto-increment
field to the following row.

The table->auto_increment_field_not_null variable is reset now right after a
row is being written in the read_fixed_length() and the read_sep_field()
functions.
Removed wrong setting of the table->auto_increment_field_not_null variable in
the read_sep_field() function.