rpc : fix cache directory initialization (#13188)

Signed-off-by: xiaofei <hbuxiaofei@gmail.com>

examples/rpc/rpc-server.cpp

@@ -304,8 +304,9 @@ int main(int argc, char * argv[]) {
         get_backend_memory(&free_mem, &total_mem);
     }
     const char * cache_dir = nullptr;
-    std::string cache_dir_str = fs_get_cache_directory() + "rpc/";
+    std::string cache_dir_str;
     if (params.use_cache) {
+        cache_dir_str = fs_get_cache_directory() + "rpc/";
         if (!fs_create_directory_with_parents(cache_dir_str)) {
             fprintf(stderr, "Failed to create cache directory: %s\n", cache_dir_str.c_str());
             return 1;