vout_subpictures.c: Fix Dereference after null check (cid 1213781)
vout_subpictures.c: Fix Dereference after null check (cid 1213781)

Fixes a bug introduced in commit 7d8714d90eaaf4f0e58685e62d14ac13d494f316

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