tsan: fix a bug in metamap

The bug happens in the following case:
Mutex is located at heap block beginning,
when we call MutexDestroy, s->next is set to 0,
so free can't find the MBlock related to the block.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@212531 91177308-0d34-0410-b5e6-96231b3b80d8