Use checked_add in frame_to_pointer

[phil-opp]

Here:

x86_64/src/structures/paging/mapper/offset_page_table.rs

Line 53 in f8a3d70

let virt = self.offset + frame.start_address().as_u64();

The physical address space is larger than the virtual one, so this can overflow on some architectures (e.g. see #289). It can also overflow when offset is large. Rust doesn't do overflow checking in release mode by default, so we should use checked_add here to ensure that an overflow always leads to a panic.