patch 9.1.0265: console dialog cannot save unnamed buffers

Problem:  console dialog cannot save unnamed buffers
Solution: set bufname before save (glepnir). Define dialog_con_gui
          to test for GUI+Console dialog support, use it to skip
          the test when the GUI feature has been defined.

Note: The dialog_changed() function will also try to call the
browse_save_fname() function, when FEAT_BROWSE is defined (which is only
defined in a GUI build of Vim). This will eventually lead to a call of
do_browse(), which causes an error message if a GUI is not currently
running (see the TODO: in do_browse()) and will then lead to a failure
in Test_goto_buf_with_onfirm().

Therefore, we must disable the Test_goto_buf_with_onfirm(), when the
dialog_con_gui feature is enabled (which basically means dialog feature
for GUI and Console builds, in contrast to the dialog_con and dialog_gui
feature).

(Previously this wasn't a problem, because the test aborted in the YES
case for the :confirm :b XgotoConf case and did therefore not run into
the browse function call)

closes: #14398

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 4a6f519..a75bfc1 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -163,7 +163,8 @@
     char_u	buff[DIALOG_MSG_SIZE];
     int		ret;
     buf_T	*buf2;
-    exarg_T     ea;
+    exarg_T	ea;
+    int		empty_buf = buf->b_fname == NULL ? TRUE : FALSE;
 
     dialog_msg(buff, _("Save changes to \"%s\"?"), buf->b_fname);
     if (checkall)
@@ -181,10 +182,27 @@
 	// May get file name, when there is none
 	browse_save_fname(buf);
 #endif
-	if (buf->b_fname != NULL && check_overwrite(&ea, buf,
-				    buf->b_fname, buf->b_ffname, FALSE) == OK)
+	if (empty_buf)
+	    buf_set_name(buf->b_fnum, (char_u *)"Untitled");
+
+	if (check_overwrite(&ea, buf, buf->b_fname, buf->b_ffname, FALSE) == OK)
+	{
 	    // didn't hit Cancel
-	    (void)buf_write_all(buf, FALSE);
+	    if (buf_write_all(buf, FALSE) == OK)
+		return;
+	}
+
+	// restore to empty when write failed
+	if (empty_buf)
+	{
+	    vim_free(buf->b_fname);
+	    buf->b_fname = NULL;
+	    vim_free(buf->b_ffname);
+	    buf->b_ffname = NULL;
+	    vim_free(buf->b_sfname);
+	    buf->b_sfname = NULL;
+	    unchanged(buf, TRUE, FALSE);
+	}
     }
     else if (ret == VIM_NO)
     {