vdpau/chroma: fix corner case of corner case when changing device
vdpau/chroma: fix corner case of corner case when changing device

To ensure the vdp_t instance pointers comparison is valid, the old and
new instances must be contemporary. Otherwise, the heap could put both
instances at the same memory address. So keep a reference to the old
device until after the comparison.

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