Fix bug https://bugs.launchpad.net/tarantool/+bug/837315

Fix bug https://bugs.launchpad.net/tarantool/+bug/837315

When write_inbox() failed, wal_write() would throw
an exception.

This had two effects:
first, a call to confirm_lsn() would confirm lsn
of the failed transaction out of order (indeed, nothing
is written, so this lsn is confirmed before lsns
of transactions which are waiting on write to complete)

second, txn_rollback() was called with fiber->mod_data.txn == 0.

The second led to an assertion failure.

The fix only fixes the second effect, so that an error
is sent to the client, and the server does not terminate.

It's not possible to come up with a serial test
case for the bug, since to fill up all inboxes one
needs many concurrent writes.