CHROMIUM: Handle exiting process in audit_log_d_path.

If the process is existing, current->fs may be NULL, so we cannot
resolve paths in audit messages.

BUG=chromium:639102
TEST=Ran on cyan.

Change-Id: Id8f98071f5f85c05c1d4257ac46fe01b5386f42e
Reviewed-on: https://chromium-review.googlesource.com/372981
Commit-Ready: Ricky Zhou <rickyz@chromium.org>
Tested-by: Ricky Zhou <rickyz@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
(cherry picked from commit 4c2b0a35a4dc9269ed229628f2fc536c3c1ce14a)
Reviewed-on: https://chromium-review.googlesource.com/375539