[FIX] core: assignment of size to new records

Upon onchange, the web client sends unchanged image fields on one2many
lines as "bin size" values, while changed image fields are sent as their
full value.  As the value is not base64-encoded, the assignment of that
value crashes.

Fix it by making assignment on new records not crash when decoding the
image: the value is simply not assigned to the field.  In the situation
described above, the record being assigned is a new record with a real
record as origin.  Because the field is actually not assigned, its value
is implicitly the value of the field on the origin record, which is
consistent with the fact that the field was not modified in the form.