Output key content is undefined.

(Output buffer content is undefined)

Faster start with MacOS:
 - saves one mount/unmount when disc does not use bus encryption.
 => lot faster startup when disc keys are already cached.

Avoid huge allocs

config.guess returns x86_64-pc-mingw64 on some msys2 installations

Signed-off-by: Christopher Degawa <ccom@randomderp.com>