misaligned pointer dereference in test nlmsg_put_attr

[riking]

https://asan.saethlin.dev/ub?crate=rsmnl&version=0.1.0

test nlmsg_put_attr ... thread 'nlmsg_put_attr' panicked at /build/src/attr.rs:78:9:
misaligned pointer dereference: address must be a multiple of 0x8 but is 0x521000017d14
stack backtrace:
   0: rust_begin_unwind
             at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:597:5
   1: core::panicking::panic_nounwind_fmt
             at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:106:14
   2: core::panicking::panic_misaligned_pointer_dereference
             at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:203:5
   3: rsmnl::attr::Attr::payload_raw_mut
             at ./src/attr.rs:78:9
   4: rsmnl::msgvec::MsgVec::put
             at ./src/msgvec.rs:211:28
   5: lib::nlmsg_put_attr
             at ./tests/lib.rs:255:13
   6: lib::nlmsg_put_attr::{{closure}}
             at ./tests/lib.rs:251:20