fixed JModelForm::checkin()

Fixing issue described in [bug #30700](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30700), related to notices for unset properties of the $table object in JModelForm (libraries/legacy/model/form.php)
Merge pull request #3262 from itbra/patch-7

[#30700] fix JModelForm::checkin()