patch 8.1.0577: tabpage right-click menu never shows "Close tab"

Problem:    Tabpage right-click menu never shows "Close tab".
Solution:   Always create the "Close tab" item but ignore the event if there
            is only one tab.
diff --git a/src/gui.c b/src/gui.c
index 2f3bafc..9160848 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -3865,10 +3865,14 @@
 {
     char_u	    string[3];
 
-    /* Don't put events in the input queue now. */
+    // Don't put events in the input queue now.
     if (hold_gui_events)
 	return;
 
+    // Cannot close the last tabpage.
+    if (event == TABLINE_MENU_CLOSE && first_tabpage->tp_next == NULL)
+	return;
+
     string[0] = CSI;
     string[1] = KS_TABMENU;
     string[2] = KE_FILLER;