decimate: check for avctx ptr before closing.

Fixes a crash init failed before setting avctx.

libavfilter/vf_decimate.c

@@ -148,9 +148,11 @@ static av_cold void uninit(AVFilterContext *ctx)
 {
     DecimateContext *decimate = ctx->priv;
     av_frame_free(&decimate->ref);
+    if (decimate->avctx) {
         avcodec_close(decimate->avctx);
         av_freep(&decimate->avctx);
     }
+}
 
 static int query_formats(AVFilterContext *ctx)
 {