Panic on unreachable code when converting object to JSON

[Razican]

In some cases, when we try to convert a specifically built object to JSON, we can get a panic because we try to convert an undefined or Symbol value to JSON.

This happens in the following 2 tests from the Test262 test suite:

• test/built-ins/JSON/stringify/value-tojson-result.js
• test/built-ins/JSON/stringify/value-tojson-arguments.js

You can reproduce it by running:

RUST_BACKTRACE=1 cargo run --release --bin boa_tester -- run -v -s {file}

Where {file} is one of the two above. The panic that gets generated is this one:

thread 'main' panicked at 'internal error: entered unreachable code: Symbols and Undefined JSON Values depend on parent type', boa/src/value/mod.rs:245:17

And the stack trace this one:

   0: rust_begin_unwind
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:495:5
   1: std::panicking::begin_panic_fmt
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:437:5
   2: boa::value::Value::to_json
   3: boa::value::Value::to_json
   4: boa::builtins::json::Json::stringify
   5: boa::object::gcobject::GcObject::call
   6: <boa::syntax::ast::node::call::Call as boa::exec::Executable>::run
   7: <boa::syntax::ast::node::call::Call as boa::exec::Executable>::run
   8: <boa::syntax::ast::node::statement_list::StatementList as boa::exec::Executable>::run
   9: boa::context::Context::eval
  10: boa_tester::exec::<impl boa_tester::Test>::run_once
  11: boa_tester::exec::<impl boa_tester::Test>::run
  12: boa_tester::main