avfilter/vf_blend: Fix AVClass

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

libavfilter/vf_blend.c

@@ -537,7 +537,7 @@ AVFilter ff_vf_tblend = {
     .name          = "tblend",
     .description   = NULL_IF_CONFIG_SMALL("Blend successive frames."),
     .priv_size     = sizeof(BlendContext),
-    .priv_class    = &blend_class,
+    .priv_class    = &tblend_class,
     .query_formats = query_formats,
     .init          = init,
     .uninit        = uninit,