src: fix compile warnings in node_url.cc

[addaleax]

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++) {
      |               ^

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/29008/

[jasnell]

Ugh so irritating these aren't reported when building on Windows.

[addaleax]

Landed in 2b1c18f