Fix one potential NULL-deref on invalid plugin cache
Fix one potential NULL-deref on invalid plugin cache

Zero-length strings are loaded as NULL. There are probably quite a few
other similar failure cases :( I have not investigated.

shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,xkfz007/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1