Make DRBG uninstantiate() and instantiate() methods inverse to each other

Previously, the RAND_DRBG_uninstantiate() call was not exactly inverse to
RAND_DRBG_instantiate(), because some important member values of the
drbg->ctr member where cleared. Now these values are restored internally.

Signed-off-by: Dr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/4402)