cdrom_id: Fix uninitialized buffers

Commit 5c6954f is actually a no-op, since static variables are already zero'ed
by default anyway (but we keep it for clarity). The real difference was that a
build with -O0 wor while a build with -O2 didn't.

Turns out that some ioctls do not actually touch the result buffer in some
cases, so we need to zero the result buffers to avoid interpreting random da as
CD properties.

https://launchpad.net/bugs/559723
https://launchpad.net/bugs/561585