allow recursice call to tcpedit_dlt_cleanup()

This is just a quick hack to prevent a double-free should
tcpedit_dlt_cleanup() call itself, which can hapen through dlt_jnpr_ether_cleanup()

Ref: appneta#813

src/tcpedit/plugins/dlt_jnpr_ether/jnpr_ether.c

@@ -164,8 +164,10 @@ dlt_jnpr_ether_cleanup(tcpeditdlt_t *ctx)
         jnpr_ether_config_t *config;
 
         config = (jnpr_ether_config_t *)ctx->encoder->config;
-        if (config->subctx != NULL)
+        if (config->subctx != NULL) {
+            ctx->decoded_extra = NULL;
             tcpedit_dlt_cleanup(config->subctx);
+        }
         safe_free(plugin->config);
         plugin->config = NULL;
         plugin->config_size = 0;