Revert "Fix potential ThreadPool UAF (mitchellh#113)"

This reverts commit dbe2291, reversing
changes made to 43c7e4b.

src/ThreadPool.zig

@@ -335,8 +335,12 @@ fn unregister(noalias self: *ThreadPool, noalias maybe_thread: ?*Thread) void {
 
 fn join(self: *ThreadPool) void {
     // Wait for the thread pool to be shutdown() then for all threads to enter a joinable state
-    self.join_event.wait();
-    const sync: Sync = @bitCast(self.sync.load(.monotonic));
+    var sync: Sync = @bitCast(self.sync.load(.monotonic));
+    if (!(sync.state == .shutdown and sync.spawned == 0)) {
+        self.join_event.wait();
+        sync = @bitCast(self.sync.load(.monotonic));
+    }
+
     assert(sync.state == .shutdown);
     assert(sync.spawned == 0);