lavfi/deshake: fix crash in uninit()

Add NULL check on avctx before calling avcodec_close() on it.

Fix trac ticket #1472.

libavfilter/vf_deshake.c

@@ -420,7 +420,8 @@ static av_cold void uninit(AVFilterContext *ctx)
     avfilter_unref_buffer(deshake->ref);
     if (deshake->fp)
         fclose(deshake->fp);
-    avcodec_close(deshake->avctx);
+    if (deshake->avctx)
+        avcodec_close(deshake->avctx);
     av_freep(&deshake->avctx);
 }