Fix GeneralPurposeAllocator crash when deallocating metadata

[Frojdholm]

Fixes #19977

This makes the following test not crash

const std = @import("std");

test "retain metadata and never unmap" {
    var gpa = std.heap.GeneralPurposeAllocator(.{
        .safety = true,
        .never_unmap = true,
        .retain_metadata = true,
    }){};
    defer std.debug.assert(gpa.deinit() == .ok);
    const allocator = gpa.allocator();

    const alloc = try allocator.alloc(u8, 8);
    allocator.free(alloc);

    const alloc2 = try allocator.alloc(u8, 8);
    allocator.free(alloc2);
}

The getMin/getMax -> remove pattern could be encapsulated in a function popMin/popMax that return the removed key.

[andrewrk]

Nice test case. Can it be added to the file as a unit test?

[Frojdholm]

Nice test case. Can it be added to the file as a unit test?

Sure, added 👍

However, I was a bit confused that the "double frees" test wasn't already failing since it's using the same setup, but with a backing allocator of another GPA.

[Frojdholm]

Test failure was a flaky windows test

[andrewrk]

Thanks!