Add a mapitest to crash on ModifyRecipients

When the number of additional properties in a recipient row is lower
than the number of additional properties set by ModifyRecipientsROP.

Adapt IMessage.c to set the proper prop_count in recipient row
by counting the added properties instead of blindly set the ModifyRecipientsROP
property count.
Fix CR-657: RecipientRow prop number is lower than ModifyRecipientsROP prop count

Instead of crashing when trying to get properties which are not
set, we set the data to NULL and at mapistore level the property count
is equal for all recipient rows.