hw/timer/slavio_timer: Remove useless check for NULL t->timer

hw/timer/slavio_timer: Remove useless check for NULL t->timer

In the slavio timer device, the ptimer TimerContext::timer is
always created by slavio_timer_init(), so there's no need to
check it for NULL; remove the single unneeded NULL check.

This will be useful to avoid compiler/Coverity errors when
a subsequent change adds a use of t->timer before the location
we currently do the NULL check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20191021134357.14266-2-peter.maydell@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>