hw/acpi/erst.c: Fix memory handling issues

- Fix memset argument order: The second argument is
  the value, the length goes last.
- Fix an integer overflow reported by Alexander Bulekov.

Both issues allow the guest to overrun the host buffer
allocated for the ERST memory device.

Cc: Eric DeVolder <eric.devolder@oracle.com
Cc: Alexander Bulekov <alxndr@bu.edu>
Cc: qemu-stable@nongnu.org
Fixes: f7e26ffa ("ACPI ERST: support for ACPI ERST feature")
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Message-Id: <20221024154233.1043347-1-lk@c--e.de>
Fixes: #1268


Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>