custom items applications= field in sessions file crashes PyHoca-CLI/GUI

Migrated from https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=809

X2Go Client obviously is able to add extra/custom items to the
applications= field in a session profile item.

We observed one case where pyhoca-gui crashed when encountering this
line in the sessions file:

applications=WWWBROWSER, MAILCLIENT, OFFICE, TERMINAL,
/usr/bin/Virtualbox %U, /usr/bin, /usr/bin/Virtualbox, Virtualbox,
Virtualbox %U

(which looks broken anyway, but PyHoca-GUI should not crash on it).