crypto: drbg - remove superflowous memsets

The DRBG code contains memset(0) calls to initialize a varaible
that are not necessary as the variable is always overwritten by
the processing.

This patch increases the CTR and Hash DRBGs by about 5%.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>