inflate/bitreader: fix unsoundness in advance()

[inahga]

BitReader::advance() is unsound: if bytes is too large, we invoke UB in self.ptr.add(bytes) by advancing the pointer out of bounds of the allocated area. This doesn't happen in practice, since all uses of advance() are bounds checking with BitReader::bytes_available(), but we can make this API safer.

Use .wrapping_add() instead, which is safe for all inputs. The docs for it claim that .add() opens up more optimization opportunities. However, the generated assembly on x86_64 and aarch64 in release mode appear to be exactly the same, so I don't think we're losing out on anything here. (The guaranteed zero-cost way to fix this would be to just make advance() unsafe and document the invariant).

[folkertdev]

yeah that's not worth the unsafe block then