skins: fix crash if input-title-format is empty
skins: fix crash if input-title-format is empty

(cherry picked from commit 541b381c676cc96b6e1967610a2077b00950b86d)

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2