patch 8.2.0214: a popup window with a terminal can be made hidden

Problem:    A popup window with a terminal can be made hidden.
Solution:   Disallow hiding a terminal popup.
diff --git a/src/popupwin.c b/src/popupwin.c
index c438000..56d04b5 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -2371,6 +2371,10 @@
     void
 popup_hide(win_T *wp)
 {
+#ifdef FEAT_TERMINAL
+    if (error_if_term_popup_window())
+	return;
+#endif
     if ((wp->w_popup_flags & POPF_HIDDEN) == 0)
     {
 	wp->w_popup_flags |= POPF_HIDDEN;