Fix thread comparison by making sure we never pass our special 'not a thread' value to the underlying implementation. Fixes PR#42918.

llvm-svn: 368916
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2e80d01fa7dc36c5f2533a363c6c0680f7896dc5
diff --git a/src/mutex.cpp b/src/mutex.cpp
index ddd36b4..1827857 100644
--- a/src/mutex.cpp
+++ b/src/mutex.cpp
@@ -181,7 +181,7 @@
     unique_lock<mutex> lk(__m_);
     if (--__count_ == 0)
     {
-        __id_.reset();
+        __id_.__reset();
         lk.unlock();
         __cv_.notify_one();
     }