Multidevice: Add a CHECK() for a null LogBuffer.

When writing logs from PA_LOG() we are occasionally seeing
a crash that appears to be caused by a nullptr getting returned
from LogBuffer::GetInstance(), which should not be possible.
This change adds a CHECK() to confirm if that is the true issue.

Bug: 1210637
Change-Id: Ice016b69de43898cbbcdcf70fbd2c4d28ff550b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3219531
Commit-Queue: Jon Mann <jonmann@chromium.org>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/main@{#932110}