Fix request #6793 PHP notice when adding values in select box field

Change-Id: I261648dde1e0e0023dcef281ea2e774f44529241
Merge commit 'refs/changes/75/2375/1' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  Fix request #6793 PHP notice when adding values in select box field

Change-Id: If60454f4ba676a97a5302e11f69b334f8f267d21
This is Tuleap 6.12.99.104

Change-Id: I67104e13d6f97712a2b3d12e2e8e7dfd984fa96c