Fix stack corruptions in special cases

Issue was caused by wrong array length used
for result of name_uiprefix_id, which shall
actually be 1 byte bugger than MAX_ID_NAME.

Reported by Sebastian Koenig in IRC.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56982 954f8c5b-7b00-dc11-b283-0030488c597c