avcodec/vp8: Fix multiple ff_thread_finish_setup() calls

Fixes Ticket3725
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vp8.c

@@ -2584,7 +2584,8 @@ int vp78_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
 
     s->next_framep[VP56_FRAME_CURRENT] = curframe;
 
-    ff_thread_finish_setup(avctx);
+    if (avctx->codec->update_thread_context)
+        ff_thread_finish_setup(avctx);
 
     s->linesize   = curframe->tf.f->linesize[0];
     s->uvlinesize = curframe->tf.f->linesize[1];