XvBA_BeginPicture: be more tolerant.

Instead of aborting on an empty buffer we just continue, since the
buffer would be freed anyway on the next line. This patch makes the
function more tolerant.

src/xvba_decode.c

@@ -623,7 +623,7 @@ xvba_BeginPicture(
     obj_surface->va_surface_status     = VASurfaceRendering;
     obj_surface->used_for_decoding     = 1;
 
-    ASSERT(!obj_context->va_buffers_count);
+    if (obj_context->va_buffers_count > 0)
         destroy_va_buffers(driver_data, obj_context);
 
     unsigned int i;