Fix defect reported by Coverity Scan.
Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking pD3D12VideoBuffer suggests that it may
be null, but it has already been dereferenced on all paths leading to
the check.
Fixes: d8206f6286 ("d3d12: Add video decode implementation of pipe_video_codec")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16714>