http_negotiate: fix CURL_DISABLE_PROXY build

`proxyuserpwd` was removed from `dynamically_allocated_data` in commit
f46385d36d.

Closes https://github.com/curl/curl/pull/13334

lib/http_negotiate.c

@@ -184,8 +184,10 @@ CURLcode Curl_output_negotiate(struct Curl_easy *data,
                     base64);
 
     if(proxy) {
+#ifndef CURL_DISABLE_PROXY
       Curl_safefree(data->state.aptr.proxyuserpwd);
       data->state.aptr.proxyuserpwd = userp;
+#endif
     }
     else {
       Curl_safefree(data->state.aptr.userpwd);