Invalid free in std.json.parseFree

[ethernetsellout]

This is what the parseFree logic currently looks like for structs & pointers:

pub fn parseFree(comptime T: type, value: T, options: ParseOptions) void {
    switch (@typeInfo(T)) {
        ...
        .Struct => |structInfo| {
            inline for (structInfo.fields) |field| {
                if (!field.is_comptime) {
                    parseFree(field.field_type, @field(value, field.name), options);
                }
            }
        },
       ...
        .Pointer => |ptrInfo| {
            const allocator = options.allocator orelse unreachable;
            switch (ptrInfo.size) {
                .One => {
                    parseFree(ptrInfo.child, value.*, options);
                    allocator.destroy(value);
                },
                .Slice => {
                    for (value) |v| {
                        parseFree(ptrInfo.child, v, options);
                    }
                    allocator.free(value);
                },
                else => unreachable,
            }
        },
        else => unreachable,
    }
}

As you can see, for structs it recursively calls itself on each field, and for pointers it deallocates them. This goes haywire when a struct field has a default value; if, for instance, a 'message' field has a default value, parseFree will attempt to free memory written in the binary:

const S = struct {message: []const u8 = "Cool default value"};

var set_message = json.TokenStream.init(
    \\{ "message": "Cool message" }
);
var leave_to_default = json.TokenStream.init(
    \\{}
);

const options = .{.allocator = std.testing.allocator};

const set_parse = try json.parse(S, &set_message, options);
const default_value_parse = try json.parse(S, &leave_to_default, options);

json.parseFree(S, set_parse, options);
json.parseFree(S, default_value_parse, options); // segfault occurs here

(https://godbolt.org/z/rYPY5or3a)

I think this issue could be fixed by adding additional checks for struct fields:

1. Check if the field is a pointer type
2. If so, check if the address is equal to the default value's address
3. If it is, skip the field