lsi53c895a: check message length value is valid

lsi53c895a: check message length value is valid

While writing a message in 'lsi_do_msgin', message length value
in 'msg_len' could be invalid due to an invalid migration stream.
Add an assertion to avoid an out of bounds access, and reject
the incoming migration data if it contains an invalid message
length.

Discovered by Deja vu Security. Reported by Oracle.

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <20181026194314.18663-1-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit e58ccf039650065a9442de43c9816f81e88f27f6)
*CVE-2018-18849
*avoid context dep. on c921370b22c
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>