sd: Fix out-of-bounds assertions

sd: Fix out-of-bounds assertions

Due to an off-by-one error, the assert statements allow an
out-of-bound array access.  This doesn't happen in practice,
but the static analyzer notices.

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <6b19cb7359a10a6bedc3ea0fce22fed3ef93c102.1560806687.git.lidong.chen@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>