vaapi: return early from ff_vaapi_render_picture() without picture

Fixes an assertion when called on uninitialized frame. Spotted after
seeking in vlc.
Fixes ticket #2461.

Tested-by: Timo
Reviewed-by: Joakim Plate

libavcodec/vaapi.c

@@ -46,6 +46,9 @@ int ff_vaapi_render_picture(struct vaapi_context *vactx, VASurfaceID surface)
     VABufferID va_buffers[3];
     unsigned int n_va_buffers = 0;
 
+    if (!vactx->pic_param_buf_id)
+        return 0;
+
     vaUnmapBuffer(vactx->display, vactx->pic_param_buf_id);
     va_buffers[n_va_buffers++] = vactx->pic_param_buf_id;