Fix #196 forbid empty linked account

Closes #196 (closed)

I added a test for this one because it addresses the same question that arose and was answered in !213 (merged), but there is no bug:

• Member accounts cannot have a zero balance, so transfer_all will not kill them.
• Non-member accounts can have a zero balance, so they will be reaped. During reaping, the AccountData will be reset to default, unlinking the identity.