ui: gtk: fix crash when terminal inner-border is NULL

VTE terminal inner-border can be NULL. The vte-0.36 (API 2.90)
code checks for the condition too so I assume it's not just a bug

Fixes a crash on Fedora 24 with gtk 3.20

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: 2b2e85d403e8760ea53afd735a170500d5c17716.1462557436.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
