hwcontext_d3d11va: fix crash on frames_init failure

It appears in this case, frames_ininit is called twice (once by
av_hwframe_ctx_init(), and again by unreffing the frames ctx ref).

Merges Libav commit 086321c612185469ebb85a1887527c8915b488b7.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
