lavfi/tinterlace: remove unnecessary NULL check in start_frame()

libavfilter/vf_tinterlace.c

@@ -165,8 +165,7 @@ static void start_frame(AVFilterLink *inlink, AVFilterBufferRef *picref)
     AVFilterContext *ctx = inlink->dst;
     TInterlaceContext *tinterlace = ctx->priv;
 
-    if (tinterlace->cur)
-        avfilter_unref_buffer(tinterlace->cur);
+    avfilter_unref_buffer(tinterlace->cur);
     tinterlace->cur  = tinterlace->next;
     tinterlace->next = picref;
 }