src: fix compile warnings in node_url.cc

Since for all affected conversions there is a preceding type check,
it’s safe to use `.FromJust()` instead.

This fixes the following compile warnings:

    ../src/node_url.cc: In function ‘void node::url::{anonymous}::ToUSVString(const v8::FunctionCallbackInfo<v8::Value>&)’:
    ../src/node_url.cc:2242:43: warning: ignoring return value of ‘bool v8::Maybe<T>::To(T*) const [with T = long int]’, declared with attribute warn_unused_result [-Wunused-result]
     2242 |   args[1]->IntegerValue(env->context()).To(&start);
          |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
    ../src/node_url.cc: In function ‘node::url::url_data node::url::{anonymous}::HarvestBase(node::Environment*, v8::Local<v8::Object>)’:
    ../src/node_url.cc:1206:34: warning: ignoring return value of ‘bool v8::Maybe<T>::To(T*) const [with T = int]’, declared with attribute warn_unused_result [-Wunused-result]
     1206 |     flags->Int32Value(context).To(&base.flags);
          |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
    ../src/node_url.cc:1211:33: warning: ignoring return value of ‘bool v8::Maybe<T>::To(T*) const [with T = int]’, declared with attribute warn_unused_result [-Wunused-result]
     1211 |     port->Int32Value(context).To(&base.port);
          |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
    ../src/node_url.cc: In function ‘void node::url::{anonymous}::ToUSVString(const v8::FunctionCallbackInfo<v8::Value>&)’:
    ../src/node_url.cc:2245:15: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     2245 |   for (size_t i = start; i < value.length(); i++) {
          |               ^

PR-URL: https://github.com/nodejs/node/pull/31689


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>