patch 8.2.4685: when a swap file is found for a popup there is no dialog

Problem:    When a swap file is found for a popup there is no dialog and the
            buffer is loaded anyway.
Solution:   Silently load the buffer read-only. (closes #10073)
diff --git a/src/popupwin.c b/src/popupwin.c
index 9c0cade..e6792c8 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1989,7 +1989,9 @@
 	new_buffer = FALSE;
 	win_init_popup_win(wp, buf);
 	set_local_options_default(wp, FALSE);
+	swap_exists_action = SEA_READONLY;
 	buffer_ensure_loaded(buf);
+	swap_exists_action = SEA_NONE;
     }
     else
     {