Unsoundness in `serialize_value` Function

[shinmao]

The serialize_value function in src/webcore/global_arena.rs contains a critical memory safety bug that causes undefined behavior when called for the first time with an uninitialized global arena.

stdweb/src/webcore/global_arena.rs

Lines 54 to 64 in 9b418d9

	pub fn serialize_value< 'a >( value: Value ) -> SerializedValue< 'a > {
	unsafe {
	let mut vec = Vec::from_raw_parts( VALUE_ARENA.memory as *mut Value, VALUE_ARENA.length, VALUE_ARENA.capacity );
	vec.push( value );
	let pointer = vec.last().unwrap() as *const Value;
	VALUE_ARENA.memory = vec.as_mut_ptr() as *mut u8;
	VALUE_ARENA.length = vec.len();
	VALUE_ARENA.capacity = vec.capacity();
	mem::forget( vec );
	JsSerialize::_into_js( &*pointer )

The function uses Vec::from_raw_parts() to reconstruct a Vec<Value> from the global VALUE_ARENA without checking if the arena has been properly initialized. When VALUE_ARENA is in its initial state (zeroed or uninitialized), this creates a Vec from invalid memory addresses, leading to undefined behavior.

Proof of Concept

#[test]
fn poc_multiple_value_types() {
    use stdweb::Value;
    use stdweb::private::serialize_value;

    let test_cases = vec![
        ("Null", Value::Null),
        ("Boolean", Value::Bool(true)),
        ("Number", Value::Number(42.0.into())),
        ("String", Value::String("test".into())),
    ];

    for (name, value) in test_cases.into_iter().take(1) {
        let _serialized = serialize_value(value);
        break;
    }
}

it triggers the following errors,

test poc_multiple_value_types ... error: Undefined Behavior: constructing invalid value: encountered 0, but expected something greater or equal to 1
  --> /home/rafael/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/unique.rs:89:36
   |
89 |         unsafe { Unique { pointer: NonNull::new_unchecked(ptr), _marker: PhantomData } }
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: this is on thread `poc_multiple_va`