avutil/hwcontext_d3d11va: correct sizeof IDirect3DSurface9

Fixes: CID1591944 Wrong sizeof argument

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Steve Lhomme <robux4@ycbcr.xyz>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 628ba061c8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/hwcontext_dxva2.c

@@ -141,7 +141,7 @@ static AVBufferRef *dxva2_pool_alloc(void *opaque, size_t size)
     if (s->nb_surfaces_used < hwctx->nb_surfaces) {
         s->nb_surfaces_used++;
         return av_buffer_create((uint8_t*)s->surfaces_internal[s->nb_surfaces_used - 1],
-                                sizeof(*hwctx->surfaces), dxva2_pool_release_dummy, 0, 0);
+                                sizeof(**hwctx->surfaces), dxva2_pool_release_dummy, 0, 0);
     }
 
     return NULL;