elm/toolbar: Fix NULL dereference waiting to happen.
elm/toolbar: Fix NULL dereference waiting to happen.

The macro returns NULL if it->prev is not valid. So we better check if prev is
not NULL before accessing it.

SVN revision: 81144

tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary