[NSE] Set L_NSE to NULL if NSE is closed (close_nse is called).

Currently NSE is only opened and closed once in Nmap so this doesn't actually
fix a visible bug.

nse_main.cc

@@ -540,5 +540,8 @@ void script_scan (std::vector<Target *> &targets)
 void close_nse (void)
 {
   if (L_NSE != NULL)
+  {
     lua_close(L_NSE);
+    L_NSE = NULL;
+  }
 }