Fix crash in destructor of detached thread

Fix issue 19314

Expectation is that after thread is detached from druntime, it becomes
responsibility of external environment to terminate it properly (for
example, by calling `join` from C/C++ code).

However, `Thread` object for that thread will still remain and will
eventually be collected by GC. Destructor tries to call `pthread_detach`
unconditionally but it is undefined behaviour if thread was already
joined before (see `man pthread_detach`).

Author: Mihails Strasuns

src/core/thread.d

@@ -596,7 +596,10 @@ class Thread
      */
     ~this() nothrow @nogc
     {
-        if ( m_addr == m_addr.init )
+        bool no_context = m_addr == m_addr.init;
+        bool not_registered = !next && !prev && (sm_tbeg !is this);
+
+        if (no_context || not_registered)
         {
             return;
         }

test/thread/src/external_threads.d

@@ -1,22 +1,50 @@
 import core.sys.posix.pthread;
 import core.memory;
+import core.thread;
+
+extern (C) void  rt_moduleTlsCtor();
+extern (C) void  rt_moduleTlsDtor();
 
 extern(C)
-void* entry_point(void*)
+void* entry_point1(void*)
 {
     // try collecting - GC must ignore this call because this thread
     // is not registered in runtime
     GC.collect();
     return null;
 }
 
+extern(C)
+void* entry_point2(void*)
+{
+    // This thread gets registered in druntime, does some work and gets
+    // unregistered to be cleaned up manually
+    thread_attachThis();
+    rt_moduleTlsCtor();
+
+    auto x = new int[10];
+
+    rt_moduleTlsDtor();
+    thread_detachThis();
+    return null;
+}
+
 void main()
 {
     // allocate some garbage
     auto x = new int[1000];
 
-    pthread_t thread;
-    auto status = pthread_create(&thread, null, &entry_point, null);
-    assert(status == 0);
-    pthread_join(thread, null);
+    {
+        pthread_t thread;
+        auto status = pthread_create(&thread, null, &entry_point1, null);
+        assert(status == 0);
+        pthread_join(thread, null);
+    }
+
+    {
+        pthread_t thread;
+        auto status = pthread_create(&thread, null, &entry_point2, null);
+        assert(status == 0);
+        pthread_join(thread, null);
+    }
 }