[libcxx] Fix thread join.pass.cpp segfault after r271475

Some pthread implementations do not like being called pthead_join()
with the pthread_t argument set to 0, and causes a segfault. This
patch fixes this issue by validating the pthread_t argument before
invoking pthread_join().

NFC.

Differential revision: http://reviews.llvm.org/D20929

Change-Id: Ief817c57bd0e1f43cbaa03061e02417d6a180c38
Reviewers: EricWF

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@271634 91177308-0d34-0410-b5e6-96231b3b80d8

src/thread.cpp

@@ -46,14 +46,17 @@ thread::~thread()
 void
 thread::join()
 {
-    int ec = __libcpp_thread_join(&__t_);
+    int ec = EINVAL;
+    if (__t_ != 0)
+    {
+        ec = __libcpp_thread_join(&__t_);
+        if (ec == 0)
+            __t_ = 0;
+    }
 #ifndef _LIBCPP_NO_EXCEPTIONS
     if (ec)
         throw system_error(error_code(ec, system_category()), "thread::join failed");
-#else
-    (void)ec;
 #endif  // _LIBCPP_NO_EXCEPTIONS
-    __t_ = 0;
 }
 
 void