[ Upstream commit 162bc7f5 ]

If you have a CPU that fails to round up and then run 'btc' you'll end
up crashing in kdb becaue we dereferenced NULL.  Let's add a check.
It's wise to also set the task to NULL when leaving the debugger so
that if we fail to round up on a later entry into the debugger we
won't backtrace a stale task.
Signed-off-by: NDouglas Anderson <dianders@chromium.org>
Acked-by: NDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: NDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>