When leaving a function use memzero_explicit instead of memset(0) to
clear stack allocated buffers. memset(0) may be optimized away.

This particular buffer is highly likely to contain sensitive data which
we shouldn't leak (it's named 'passwd' after all).
Signed-off-by: NGiel van Schijndel <me@mortis.eu>
Acked-by: NHerbert Xu <herbert@gondor.apana.org.au>
Reported-at: http://www.viva64.com/en/b/0299/
Reported-by: Andrey Karpov
Reported-by: Svyatoslav Razmyslov
Signed-off-by: NSteve French <steve.french@primarydata.com>