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_plugins.c

@@ -442,16 +442,18 @@ void
 tcpedit_dlt_cleanup(tcpeditdlt_t *ctx)
 {
     tcpeditdlt_plugin_t *plugin;
+    tcpeditdlt_plugin_t *plugin_orig;
 
     assert(ctx);
 
-    plugin = ctx->plugins;
+    plugin_orig = plugin = ctx->plugins;
+    ctx->plugins = NULL;
     while (plugin != NULL) {
         plugin->plugin_cleanup(ctx);
         plugin = plugin->next;
     }
 
-    plugin = ctx->plugins;
+    plugin = plugin_orig;;
     while (plugin != NULL) {
         tcpeditdlt_plugin_t *plugin_next = plugin->next;
         safe_free(plugin);