patch 8.0.1223: crash when using autocomplete and tab pages

Problem:    Crash when using autocomplete and tab pages.
Solution:   Check if the current tab changed. (Christian Brabandt, closes
            #2239)
diff --git a/src/popupmnu.c b/src/popupmnu.c
index 418f0ca..ec75281 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -566,6 +566,7 @@
 		&& vim_strchr(p_cot, 'p') != NULL)
 	{
 	    win_T	*curwin_save = curwin;
+	    tabpage_T   *curtab_save = curtab;
 	    int		res = OK;
 
 	    /* Open a preview window.  3 lines by default.  Prefer
@@ -653,8 +654,13 @@
 		    curwin->w_cursor.lnum = 1;
 		    curwin->w_cursor.col = 0;
 
-		    if (curwin != curwin_save && win_valid(curwin_save))
+		    if ((curwin != curwin_save && win_valid(curwin_save))
+			    || (curtab != curtab_save
+						&& valid_tabpage(curtab_save)))
 		    {
+			if (curtab != curtab_save && valid_tabpage(curtab_save))
+			    goto_tabpage_tp(curtab_save, FALSE, FALSE);
+
 			/* When the first completion is done and the preview
 			 * window is not resized, skip the preview window's
 			 * status line redrawing. */