LinGui: fix crash caused by bug in newer versions of gtk2

GtkCombobBoxText is not initialized correctly by gtk2.  Reading out it's
current value fails and returns NULL.  So explicitely initialize the
text column value and guard against future bugs by checking return value.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5231 b64f7644-9d1e-0410-96f1-a4d463321fa5