crypto: drbg - remove unnecessary sanity check for shadow state

During creation of the DRBG shadow state, it is ensured that the DRBG
state structure is already allocated. Thus, a sanity check for verifying
that the structure is allocated is removed.

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