miri does not detect reading undefined bytes if simple unions are involved

[oli-obk]

http://play.rust-lang.org/?gist=99912c4c87d4d676817c8a7263289c9b&version=undefined&mode=undefined

shows an example where miri returns zeroes instead of erroring because of undefined bytes being read.

[oli-obk]

not really unsound, because you already were doing UB via unsafe code. Just miri should be better at detecting it