Fix #4471: Undefined index warning with referenced column.

Hi,
I did this PR for https://sourceforge.net/p/phpmyadmin/bugs/4471/ .
The problem is that I can't reproduce it on 4.2, only on master… It works on master. But I would appreciate if someone can confirm this is ok on 4.2 please.
(If possible, I would like to try to merge it in 4.2 and master.)
Thanks.