[sanitizer] Fix log_path behavior with StopTheWorld.

Summary:
Fix race on report_fd/report_fd_pid between the parent process and the
tracer task.

Reviewers: samsonov

Reviewed By: samsonov

CC: llvm-commits, kcc, dvyukov

Differential Revision: http://llvm-reviews.chandlerc.com/D2306

llvm-svn: 196385