tsan: fix trace initialization during thread id reuse

The current code leaves the first event in the trace part uninitialized
(from the previous thread). It can cause unpredictable behavior
during stack/mutexset restoration.
Initialize the first event to a fake harmless memory access.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224834 91177308-0d34-0410-b5e6-96231b3b80d8