tsan: don't crash with NULL deref during reporting

tctx==NULL crash observed during deadlock reporting.
There seems to be some bugs in the deadlock detector,
but it is still useful to be more robust during reporting.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224508 91177308-0d34-0410-b5e6-96231b3b80d8