[tsan] Do nothing in ScopedInterceptor's destructor if thr is not inited.

Summary:
It is part of http://reviews.llvm.org/D15301, but missed when I committed
that patch.

Reviewers: kubabrecka, kcc, eugenis, llvm-commits, dvyukov

Differential Revision: http://reviews.llvm.org/D16235

llvm-svn: 258021