mbox: Removed assert that caused dsync to crash. This is valid:
mbox: Removed assert that caused dsync to crash.
This is valid:

1. dsync opened an internal transaction and locked mbox
2. dsync opened an external transaction, which didn't lock mbox
3. dsync committed internal transaction
<assert crash fixed>
4. dsync locks mbox with external transaction

Distrotech/dovecot,Distrotech/dovecot,Distrotech/dovecot,Distrotech/dovecot,Distrotech/dovecot