dispatch: explicitly cast to avoid truncation warning

Add an explicit cast from `void *` to `uintptr_t` before truncating the
context to the `dispatch_flags_t`.  Add an assertion to ensure that we
are not dropping something.

Author: compnerd

src/queue.c

@@ -794,7 +794,10 @@ _dispatch_async_redirect_invoke(dispatch_continuation_t dc,
 {
 	dispatch_thread_frame_s dtf;
 	struct dispatch_continuation_s *other_dc = dc->dc_other;
-	dispatch_invoke_flags_t ctxt_flags = (dispatch_invoke_flags_t)dc->dc_ctxt;
+#if DISPATCH_SIZEOF_PTR == 8
+	dispatch_assert(((uintptr_t)dc->dc_ctxt >> 32) == 0);
+#endif
+	dispatch_invoke_flags_t ctxt_flags = (dispatch_invoke_flags_t)(uintptr_t)dc->dc_ctxt;
 	// if we went through _dispatch_root_queue_push_override,
 	// the "right" root queue was stuffed into dc_func
 	dispatch_queue_global_t assumed_rq = (dispatch_queue_global_t)dc->dc_func;