tool: call PL_ArenaFinish() on exit if NSPR is used

This prevents valgrind from reporting still reachable memory allocated
by NSPR arenas (mainly the freelist).

Reported-by: Hubert Kario

src/tool_main.c

@@ -210,9 +210,12 @@ static void main_free(struct GlobalConfig *config)
   convert_cleanup();
   metalink_cleanup();
 #ifdef USE_NSS
-  if(PR_Initialized())
+  if(PR_Initialized()) {
+    /* prevent valgrind from reporting still reachable mem from NSRP arenas */
+    PL_ArenaFinish();
     /* prevent valgrind from reporting possibly lost memory (fd cache, ...) */
     PR_Cleanup();
+  }
 #endif
   free_config_fields(config);