Fix improper unaligned reads in value_from_ptr

[RealKC]

copy_nonoverlapping documents that its pointer must be aligned for a valid read, otherwise the call will be UB.

This patch will fix that, and I believe it will fix #269. (I tried @igor-petruk's oatbar at commit igor-petruk/oatbar@baf1627 which is what he linked in the issue, changed the Cargo.toml to depend on my local copy that had this change and did cargo run, and I didn't get a crash.)

This was only somewhat recently noticed because Rust started checking these preconditions in Rust 1.78.

As some notes:

• should we add debug_assert!(!ptr.is_null()) to this function?
• is there a point in having this function now vs modifying the code generator to call read_unaligned?

[rtbo]

Thanks for that.
I think indeed it is correct.

Regarding your notes:

• The pointer passed to value_from_ptr are the one returned by the libxcb API. Assertion should not be needed.
• Calling read_unaligned directly from code generator is also fine.