video: fix dangling pointer issue

The function video_decode_and_filter(), called between initializing the
local vf variable and using it, can actually destroy and recreate the
filter. Thus, the vf variable turns into a dangling pointer if that
happens.

Could be observed with: --hwdec=vda --deinterlace=yes --vf=yadif

(Also happens with vdpau/vaapi.)