web_service: avoid undefined behavior assert of std::string::back (#7347)

Co-authored-by: BuildTools <unconfigured@null.spigotmc.org>

src/web_service/web_backend.cpp

@@ -30,7 +30,7 @@ struct Client::Impl {
             jwt = jwt_cache.jwt;
         }
         // normalize host expression
-        if (this->host.back() == '/') {
+        if (!this->host.empty() && this->host.back() == '/') {
             static_cast<void>(this->host.pop_back());
         }
     }