Arena allocator broken

[shawnl]

This code crashes with null pointer dereference:

const std = @import("std");
const heap = std.heap;

pub fn main() !void {
    var direct_allocator = heap.DirectAllocator.init();
    defer direct_allocator.deinit();
    var direct = direct_allocator.allocator;

    var arena_allocator = heap.ArenaAllocator.init(&direct);
    defer arena_allocator.deinit();
    var arena = arena_allocator.allocator;

    var buf = try arena.alloc(u8, 10);
}

[andrewrk]

This is #591