megasas: avoid NULL pointer dereference

While in megasas_handle_frame(), megasas_enqueue_frame() may
set a NULL frame into MegasasCmd object for a given 'frame_addr'
address. Add check to avoid a NULL pointer dereference issue.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Fixes: https://bugs.launchpad.net/qemu/+bug/1878259


Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Acked-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20200513192540.1583887-3-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>