Field_new_decimal::store_value(const my_decimal*, int*)

Analysis
========
When rpl applier is unpacking a before row image, Field::reset() will be
called before setting a field to null if null bit of the field is set in
the row image. For Field_new_decimal::reset(), it calls
Field_new_decimal::store_value() to reset the value. store_value() asserts
that the field is in the write_set bitmap since it thinks the field is
updating.

But that is not true for the row image generated in FULL_NODUP
mode. In the mode, the before image includes all fields and the after
image includes only updated fields.

Fix
===
In the case unpacking binlog row images, the assertion is meaningless.
So the unpacking field is marked in write_set temporarily to avoid the
assertion failure.