avfilter/vf_colormatrix: make sure the number of threads is even for yuv420p

Fix crash with for example:
  ffmpeg -f lavfi -i testsrc=568x320 -threads 3 -vf format=yuv420p,colormatrix=bt709:smpte170m -f null -

libavfilter/vf_colormatrix.c

@@ -416,7 +416,7 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
                                FFMIN(in->height, ctx->graph->nb_threads));
     else if (in->format == AV_PIX_FMT_YUV420P)
         ctx->internal->execute(ctx, process_slice_yuv420p, &td, NULL,
-                               FFMIN(in->height, ctx->graph->nb_threads));
+                               FFMAX(1, FFMIN(in->height, ctx->graph->nb_threads) & ~1));
     else
         ctx->internal->execute(ctx, process_slice_uyvy422, &td, NULL,
                                FFMIN(in->height, ctx->graph->nb_threads));