Add explicit zeroing to "envp" array in device 'show' method

As Stephen Hemminger says, this is a "belt and suspenders" patch that
zeroes the envp array at allocation time, even though all the users
should NULL-terminate it anyway (and we've hopefully fixed everybody
that doesn't do that).

And we'll apparently clean the whole envp thing up for 2.6.24 anyway.

But let's just be robust, and do both this *and* make sure that all
users are doing the right thing.

Acked-by: Stephen Hemminger <shemminger@linux-foundation.org>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>